When It Isn’t Best to Switch to Synthetic Oil

You see it all the time on almost every car forum.

Conventional-Synthetic-Oil-01The first question almost every new person asks when they get their “new” car is what to do with it first. And the response is pretty much unanimous: bring the car or truck up to date in maintenance and if you aren’t using synthetic oil, switch as soon as you can. But is switching to synthetic oil always the best thing to do with a car that has 100K mile or more? Coming from personal experience, I no longer feel like switching an older car to synthetic oil is always sound advice.

There happened to be two instances in my life when I switched my new to me used car over to synthetic oil and regretted it soon after. Both times, the cars had just over 100K miles on them and both were dealer maintained with service records to back it up.

The first time I ran into an issue with synthetic was a few years ago. I just sold my Volvo 940Ti to a friend that was in need of a reliable car for a good price. A few weeks later, I was inching to get my hands on another Volvo, so I bought a well maintained, 1995 Volvo 850 T5 Wagon. The Volvo was in excellent shape, dealer maintained, and needed nothing. I had all the service records to back up what the previous owner was telling me.

The only thing I did was the same thing I do on all my used cars. I changed the oil to synthetic and threw on a new filter. Everything went well and nothing was leaking. About two months later, I started to see a small drip on the ground under my car at work. I knew it was coming from my car because I was the only person that parked there. When I got home, I jacked up the 850 and saw that the oil was coming from the rear main seal.

So, what caused the RMS to go? Putting the synthetic oil in the 850 cleaned out the engine so fast that it clogged up the oil separator box and the built up pressure popped the seal. But can I say with 100% accuracy that switching to synthetic oil was the cause? No, I can’t. But it seems to be the most likely choice being that my car was at 100% before the oil switch. After I fixed the seal and replaced the PCV system, I went back to very good dino oil and never had another issue. But this situation didn’t stop me from being on the synthetic band wagon.

What really did me in was what happened over the summer with my latest used car purchase. This past summer, I picked up a well maintained, but not running 1999 Mazda Miata. The pervious owner was my neighbor and she was moving to Pittsburgh. Towing a non-running car was out of the question, so I got her to sell it to me cheap. It had just over 100K miles just like the 850 and I was able to get the Miata running perfectly over a weekend.

Because of my past history with synthetic, I just changed the oil with Castro. I drove it without any issues for a few months just to make sure that everything was perfect and nothing was leaking. In August, I decided to switch the Miata over to synthetic Mobil 1, just like I did with the 850. Low and behold, about a couple weeks later, I noticed the oil started to get low, but this time, it was not leaking anywhere. I went over the whole car and could not find a leak at all.

The engine compression was still 160 across all cylinders and there was no pressure build up in the engine. After scratching my head for a couple weeks, I gave up and went back to Castro dino oil like the car had when I brought it. About a week later, the lower oil issue stopped. Both the Mobil 1 and Castro were both the same weight oil and everything. So I went on the Maita forum and it looks like this happens with these cars. They just don’t like synthetic oil after being on dino for so long.

Synthetic oil isn’t the end all for all cars. It is best to research your model of automobile and see what other members have done and what issues have come up when switching to synthetic.

Let me know down below what your take on synthetic oil is or if you’ve ever had an issue like I have had after making the switch.

About The Author: Andrew Owendoff

AndrewAndrew has been working with us at FCP Euro on and off for many years. He brings to the team over a 10 years of automotive experience and has worked on many vehicles from Volvo, Mazda, and Ford. Beside doing normal maintenance, he his also an avid modder and has done everything from exhaust systems and stereos to tuning and turbo swaps. He can be reached at aowendoff@outlook.com


Categories: Advice, Tech Talk

131 replies

  1. I don’t raelly buy into the idea that synthetic oil “made it leak”. If you had crud in the engine, THAT is what clogged the PCV and made the RMS leak. The crud may have built up and blocked a passage and you’d have no lubricant to the bearings.
    Or it might have let go at the next oil change and plugged the PCV then.
    Or the PCV was already mostly plugged and it just happen to push the RMS over the edge shortly after your synthetic change.

    I may be wrong and don’t mind being corrected, but I can’t think of an instance where the proper synthetic oil isn’t better than proper regular oil.

    In my old Mopar days, it was common to say “don’t switch to synthetic, it’ll leak”. My thoughts were that if it leaks, I fix the leak. The oil doesn’t matter much. Thicker oil (synth or regular) might not seep past a very small crack in a gasket, but when it gets large enough, it’ll leak whatever fluid contacts it.

    In other words, a leak is a leak, and I’ll fix it when it happens.

    • FCP,, You need a NEW Expert!! This guy is not Knowledgeable at ALL! He needs to get Educated. His problem was due to Previous Service was NOT Adequate!! Period!!

    • I have to agree with Robdiesel, switching to synthetic is a good thing for the engine. Yes, there can be oil leaks, but most of them will go away as the seals are once again being lubricated and expand. The sludge that was there helped dry out the seals, so I suggest not ‘fixing’ minor leaks and just wait a while.
      Of course, the oil separator box needs to be cleaned out from the years of sludge buildup, but that would be needed anyway.

      Dealer serviced doesn’t mean much to me, as they only do minor things and not the needed preventative maintenance that should be done. It is like flushing the ATF, they never do that as PM.
      Clean up your engine and reduce wear by changing to synthetic. Of course, you can change the filter with out dumping the oil if you suspect that the filter is clogged.

    • Never had a problem with synthetic oil. Put close to 500k kilometers on a 1991 volvo 940gle. Bought it wit 208k and sitched to Mobil 1.Always serviced it myself, never a leak. Crankcase ventilation system needs to be maintained.

    • I could not “post” so I will reply
      I agree that some expert did not evaluate the experience properly.
      I have had 2 Volvo 850’s and a v70. I lost 2 engines and finally determined the baking of the regular oil caused the problem. The breather boxes plugged up with “ash” from the baked oil. at 100,000 miles changing the breather is a must on an 850, synthetic oil will not bake and the Bible (owners Manuel) states Volvo engines are designed for synthetic oil, meaning using anything else enters into an untested realm. All engineering and testing was done with synthetic oil. My suggestion at this point is if Regular oil was run in a Volvo–avoid it like the plague

      • what are you talking about? Volvo’s owner’s manual states that they only put synthetic if you recommend it. Regular

  2. This is true with the Volvo situation for sure, but if I stayed with the oil that was always used, it might not have broken up the gunk and may not have caused all the issues I ran into. Also, it doesn’t explain the issue I ran into with my Miata.

    There is no doubt that synthetic oil is better then conventional oil. This blog post wasn’t about whether one is better then but more about whether the cost of switching to synthetic is worth the cost of the repairs that may follow in certain situations.

    • I had the same think happen on my ’97 Volvo 850 T5. I switched to Mobil 1 at 171,000 miles. After a few months I developed an ever worsening leak at the RMS but I did not associate it with Synth Oil. After reading several articles on the folly of switching to synth on high mileage vehicles, I went back to Dino oil with the High Mileage additive which I think is a seal conditioner. After the second oil change the leak has almost disappeared. I’m sure other oils are as good, but I used Valvoline Maxlife 5w-30 for High Mileage engines. Thanx Valvoline! FWIW, I also use Maxlife ATF and am very happy with it

      • I am an ASE Mater Tech,, Current. I also have the L-1 Advanced Cert. The Synthetic oil did not and Will Not cause the Problem you described. The oil separator box and the crankcase vent system on Volvos have always been a poor system. The Leak started Because the Dealer Did NOT service the car Properly. Seen it Far too many Times!! PERIOD!!

      • Hi Thomas,
        You certainly seem well qualified to analyze my situation. So, I switched from Dino to Synth ad 171k and developed a small leak within a few months. The leak got worse over the next 4 years to the point of leaving about an 9 inch diameter oil spot overnight. I switched back to the above mentioned Dino at 220k miles and the leak has slowed to 2 or 3 drops overnight. Same oil viscosity for both oils, 5w-30 as the factory recommends. I have not and will not replace the RMS. I have done nothing to the breather system. Maybe with your impressive credentials you can tell me what’s going on in my beloved old 850 Turbo? BTW, it has had excellent service since new, OCI always in the 3000-3500 range with new Purolator 1 or Wix filters every time.

      • I also had the same thing happen on three (count ’em, three!) high mileage 960s. You would think I would have learned my lesson after the first two. Anyway, I am convinced on these cars it had nothing to do with the breather box (not clogged), it’s just the brittle nature of the RMS is somehow kept mostly intact by the crud of the dino oil. Lesson (well) learned .

      • 1984 Volvo 240 with 336,000 miles. My oil breather box was quite clogged a while back…maybe due to using dino oil for decades so I replaced it. My car still loses oil but not much – maybe a 1/2 quart every couple thousand miles. I use G-Oil synthetic.

    • Just curious to what weight castro you used in the miata with over 100,000 as for the engine lubricant getting the proper additives

    • Andrew Owendoff – “There is no doubt that synthetic oil is better then conventional oil. This blog post wasn’t about whether one is better then but more about whether the cost of switching to synthetic is worth the cost of the repairs that may follow in certain situations.”

      My argument is that the repairs would need to be done either way, either as preventative service or after a failure. The synthetic oil might show these failures quicker, but rather that near the home than on the road cross-country.

      My “Italian tune-up” on my old ’96 965 revealed a leaking radiator hose. I wouldn’t say the “tune-up” was detrimental. I think it saved me from leaking heater/coolant hoses on my cross-country trip by revealing faults earlier and in a controlled environment.

      Leaks are leaks, crud is crud. It needs to be fixed/cleaned either way.

    • Not to base on you here, but keeping with the regular oil would only have caused different problem, galled valve guides and the famous (but not acknowledged by Volvo) burned exhaust valves
      Thanx for you effort

    • Hi Andrew:

      I spoke to an industrial lubricant expert last year. She told me the only countries where ‘real’ synthetic engine oil are sold are Germany and Japan. Other countries rate their oil, synthetic, semi or dino based on their compliance to oil standards ratings. What’s your opinion on this this?

  3. It is best to stay away from synthetic oil in higher mileage engines. Seals get hardened and synch can make its way through them easier than dyno. Synthetic oils are fantastic but the seals really need to be in good condition.

  4. Bad seals are bad seals. Period. Fix them. Same with Gaskets. I run M1 in two 350,000 mile 8-Valve Volvo’s…. One 150,000 8-valve Volvo, one 175,000 mile 16-Valve Volvo, and a 2012 Fiat 500. In every case it was a drain and fill.

    FYI, Oil separator boxes separate (condense) oil from crankcase vapors. As such it’s *impossible* for crud to make it up there. That sludge in the PCV was there from years of poor dino oil.


    • +1
      It is very bad advice to say ‘stay away from synthetic oil in higher mileage engines’. Sure it may have exposed a weak point in your Volvo engine that was probably going to fail at some point in the near future anyway. But it also corrected some very important other weak points, notable built up sludge, varnish, and other gunk. It also provides better friction protection over longer oil change intervals and may allow your engine to function properly for a longer time while allowing you to change oil less often.

      This is not to say that conventional oils are bad, they work just fine in most non-turbo applications. For example you’ve found a conventional oil that works well in your Miata and a synthetic that doesn’t, that’s fine. It doesn’t mean that all synthetics will be bad in all Miatas and all conventionals will be good.

      A sample size of TWO anecdotes does not support the conclusion you make in this article. To put it simply, this is STUPID advice.

  5. Did this on a 2008 GMC pickup. The truck had 53K when I bought it, fresh dino oil. Took it back once for a 4K dino oil change at the dealer. At 60K I switched to synthetic, with a good filter. The oil pressure reading started to continually drop at about 67K. The syn oil was dark, darker than I expected. Changed the filter, opened it up to find a LOT of nasty stuff on the pleats. Oil pressure back to normal with a new filter, same oil. It appears the syn oil is REALLY good at cleaning stuff up… so good it plugged the filter. To be fair, the filter on this truck is way too small for any vehicle, sort of a joke. But still, I did get a lot ofloose stuff out of the engine or in suspension inthe oil. The comments about plugging in the original post seems correct.

  6. The problem with older vehicles is the seals are made out of a rubber based material that was designed for fossil based oil. The later model vehicles use seals based on PVC technology that seal better and withstand different grades and types of oil. I have had the same experience on my vehicles by switching from fossil based oil to synthetic. There seems to be a lot of seepage with synthetic that is not evident with fossil based oils. I have had many customers with the same complaint and there is an article in the archives of API that addresses this point.

    • John, you make a distinction between what you call fossil based oil and synthetic oil. Please educate yourself on what synthetic motor oil is. It is a hydrocarbon based oil that starts out similar to regular motor oil but is then re-synthesized so that it has higher lubricity, and many additional characteristics related to heat tolerance and lifespan. In the end, it is motor oil. I’m not saying that it will not interact with everything it comes in contact with like seals exactly the same as traditional motor oil, I’m merely pointing out that all synthetic motor oils start out as fossil based hydrocarbons. With respect…

      • Be careful

        “Synthetic oil” has 2 meanings… one related to the manufacturing process and another based on lawyers and advertisers. “Real” synthetic oil is manufactured in a chemical process from a pure base chemical. It is NOT highly refined mineral oil. Read “all about oil” on google. It is long, based on motorcycle engines, but there is a lot of non-proprietary information there on every kind of oil and most every engine, including diesels.


        Reading this document shows true synthetics certified as Group 4 or Group 5 are man-made, manufactured from PAO or Ester bases. this is not so with “advertising synthetics” which use Group 3 mineral oil as the base.

        One mineral oil manufacturer (Castrol, not Castro) advertised their Group 3 mineral oil (SynTec) as “synthetic”. Mobil took the issue to court (lawyers with too much time on their hands) and… lost! Mobil 1 is a Group 4 oil. Castrol got a ruling they could use “synthetic” to describe their highly refined Group 3 mineral oil. Go figure.

        Plenty of stuff online to waste hours and hours on this subject.

        Final point is… buyer beware! It is not all as it seems!

      • You are not entirely correct in your remarks. Some synthetics consist of 100% ester (plant) based feed stocks and therefor contain no hydrocarbon / fossil base. With respect…

  7. About 15 or 20 years ago I changed the oil in my Toyota Cressida to synthetic. After a couple weeks, I had to have the engine seals replaced. I thought it was just bad luck but a few years after that I bought a Toyota Supra. I changed to synthetic oil, and again both seals started leaking. I switched back to regular oil and the leaking stopped after a while. I have not used synthetic oil since. I recently bought a 2012 Toyota Yaris though and I am thinking of trying synthetic one more time.

    • Backs up my experience with a variety of older vehicles. Never knew the reason but had the same results. My father often said “use the same oil that was run before if you can find out what that was!” As for the purist who say replace the seals, etc. “Show me the money!”

  8. “They just don’t like synthetic oil after being on dino for so long.” … That sounds like real solid science and not AT ALL like anecdotal rhetoric that has been passed back and forth across a forum.

    Both those cars were DUE for RMS replacement at their respective ages. How could you possibly say that the car “was 100%”?

    And of course the leaks stopped after you switched back.., but not because you switched back. It’s because you replaced the seal! It’s called maintenance!

    The real question is whether or not the synthetic does a better job at lubricating a motor, and the answer is yes. The next question is whether it is worth the additional cost, and that should be left up to the owner to decide.

    If you want to learn more about oil and how real analysis is done, read this:


    • What a bunch of SELF PROCLAIMED EXPERTS … Just like with Apple products claiming Iphone 5 is far superior to the OLD FASHIONED 3rd generation versions … This is what being influenced by Asia has done to American consumers . EVERYTHING NEW is GOOD and Old Fashioned is Inferior … I know a lot about high mileage engines and good lubrication also yet I am only pointing out how childish and pointless it is to get on here and try to impress the masses with your SUPERIOR KNOWLEDGE … Ha !
      I Work on many Volvos and BMWs but never claim to KNOW IT ALL …
      Many of you call yourselves MASTER TECHS and so-forth !
      Please !

  9. True here with the same experience. And I also use “high mileage” oil in my 200K+ 1994 850T volvo, because it has more additives in it, at about the same level as the oil that was for sale when the car was made. Modern dino oil has a lower additive content. I also had a small RMS leak when using “regular” oil, but it stopped when I went to a high mileage oil.

  10. I have 1.8t Passat, so it is one of the ten commandments of Passat owners to use approved synthetic oil. I got my car at 197.000 miles and switched to semi-synth (synth blend) and then full synth. I have now 277.000 miles with no leak.

  11. Synthetic motor oils have more powerful detergents in them than do conventional oils. They have to in order meet the huge oil change intervals of modern engines. This is what causes an engine previously serviced with conventional oil to “sludge up” it’s PCV or flame trap crankcase breather system. It’s very similar to what happened to many engines decades ago when detergent oils first became available.
    Synthetic oils also, by design flow better and reduce friction better than conventional oil. This contributes to oil leakage and consumption on an engine previously serviced with conventional oil and then switched late in its life.

  12. As I see from the replies here, it isn’t a cut and dry subject. I say “whatever works” and agree that it is a good idea to check with forums for a particular vehicle before switching. BTW, I’m not familiar with Castro motor oil. I do know about Castrol. 🙂

  13. Years ago I switched to synthetic oil on both my Volvo 850’s (a 1994 & 1997) and have never had a leak. Both do use a little oil however and I keep a spare quart of synthetic in the trunk to top it off every couple of gas fillups if needed. But no leaking of oil.

    • What mileage were your 850s when you did the switch, and what brand did you switch to? Maybe Mobil 1 in particular rather than Synthetics in general are the problem? I know Mobil 1 is a great oil, but maybe there is a problem with it and old seals? Maybe there is a problem with Volvo seals in particular?

  14. Sorry to be “that guy” but I’m “itching” to say that Castol makes oil and Castro leads a very pleasant Caribbean nation.

    I had separator box problems on my ’97 850 twice now (390,000Km), the first time it blew the RMS and oiled the clutch, the second time it ejected the exhaust side cam seal. The first time happened one year when the car only drove around the city and the temperature went down to -40. The second time was a couple of months ago when the temperature went down to -35. The separator box didn’t have any caramel in it and the block ports were clear. The hoses, especially the small bore one, seemed to be the real issue. It doesn’t take much goop to plug a long, thin, tube.

    When the system got plugged up the first time, the shop that did the work said that the separator needed replacing. They only replaced the box, not the hoses. The second time I did the work myself and, after replacing the whole crankcase ventilation system, the car runs smoother, doesn’t have any oil leaks, and the oil consumption is way down.

  15. I’ve always heard that switching an old car that always used mineral oil to synthetic will break down old oil and mineral deposits that will cause leaks. Also that conventional oil causes seals to expand more than synthetic oil does. So if you switch to synthetic oil, your seals won’t be expanded as much as previously with conventional oil, so it will leak.

  16. I wish I had the time, money, and know-how to rebuild my engines on a whim. If it leaks, fix it…sure. Bad seals are bad seals, fix them…right. But if it has no problems with dyno oil than it doesn’t have a leak…and the seals aren’t bad, so there is nothing to fix. Sure, it may need to be fixed eventually but there is a really big area called “normal usage” that comes between “brand new” and “needs to be fixed immediately.” That is the area most of us are driving in so if it doesn’t have an issue with dyno oil, why do something that might make it start having problems? The benefits of synthetic oil aren’t big enough to warrant that risk, especially once my cars are over 100,000 miles.

    That being said, I monitor my oil regularly with a full oil analysis on every oil change so I have a very good idea if I have any abnormal wear issues. This offsets one of the benefits of synthetic oil, which is that it will make your engine last longer. With sythetic oil and no analysis, I am just trusting it is doing the job…but still, have no clue if something is wearing abnormally for a non-oil related reason. With dyno oil and an oil analysis, I actually know what is wearing and how the engine is doing. I’d rather run dyno and know what is going on than synthetic and not know. I could do both but the cost savings of running dyno over synthetic is enough to offset the oil analysis cost. (I run 10K oil change intervals with dyno and because I run a turbo likely wouldn’t go any longer than that with synthetic anyways, so longer oil change intervals is not a benefit synthetic would bring me.)

    • Houston, I’m glad you brought up oil analysis. If you hadn’t I would have. I wonder if we can get the oil analysis company mentioned in the blog post on that subject to weigh in on the subject of dino vs. synth, what happens when you change from one to the other, etc.

    • Man! You guys are brutal about the Castro oil!!
      You’ve never heard of it???
      After an oil change your car will consistently pull to the left!!

  17. I bought my ’05 V50 T5 with 28K miles and have always run synth oil. The car now has 140K miles fairly gently driven and 5K mile oil change intervals. I do notice that it uses a little more oil lately but not alarming (no leaks). It seems somewhat dependent on the brand (Mobil 1 or German Castro) and viscosity.

    Given that the car has had synthetic since 28K miles and mow has 140K miles, should I continue using synthetic?

    • My understanding is that if your car used Synth from day one then you should use it forever. The problem seems to be the switch from Dino to Synth. I have found mucho good info from the hyper oil nerd forum bobistheoilguy.com

  18. Synthetic doesn’t do any cleaning that would cause this.
    Synthetic oil has molecules of consistent size, which is why it works better. Have consistently smaller molecules allows it to seep into places conventional oil cannot.

    Synthetic doesn’t cause leaks, it is just better at getting out of them. If you really think about it, its a good testament to how good a job it does when the engine is not leaking

  19. Have a ’93 volvo 245, Mobil 1, put an oil gauge on it, cheap plastic tubing cracked, wife drove car till oil light came on. Then turned car around and drove it home 4 miles no oil. That was 20,000 miles ago. Believe syn oil saved it.

  20. I got a ’93 Volvo 940 with 328K km (just over 200k miles) I switched to Mobile 1 and have had no issues so far. Have driven about 5000 km

  21. Those of you who have had issues switching between syn and dino may be running with old info. Originally, synthentic oils were formulated without seal conditioners. Engine seals harden as they age. Seal conditioners soften the seals and actually make them swell. If an aged engine seal returned to its hardened state, any oil could cause leaks. But since synthetic oils flow easier when cold, that only compounded the problem. The engine parts on a cold engine contract. Add in hardened seals and a thinner oil and guess what—you get leaks.
    But today’s synthetic oils have the same seal conditioners as conventional oil. So if someone tells you that switching to synthetic will cause your engine to leak, tell them their information is outdated by about 25 years.

  22. I purchased a Subaru Outback with the H6 engine and 163k miles and had no records but I always used synthetic oils when possible.
    Not long after that I started getting telltale leaks on the driveway and it was coming from the horizontally opposed head valve covers. I did temporary cheat by tightening the bolts but eventually had to replace them and have been fine since and am now at 207k.

    So synthetic can cause issues! But I will still use it because I’m sure it helps the engine last longer.

  23. I used Redline on my 07 V70 after 4 dealer conventional changes, 33,000 miles. Oil was very dark at initial OC. After 3 OC’s it became the normal dark amber color at 7,000 miles. I use only full synthetic, usually a 0W30. From comments on the Volvo forums, BITOG and 2 local dealers I believe white block engines are best run on full synthetic.

    I will inherit my parents 93 245 with 77,000 miles this summer. It has a considerable amount of sludge due to a life time of short trips and convention oil changes. I’ll change all gaskets, seals, oil separator, PCV etc and the filter every 1,500 miles as part of the engine baseline plan. Afterwards I’ll run Rotella T5 10W30 as I don’t believe red block engines benefit from synthetic. I plan to add OP and OT gauges to monitor.

    Synthetic oil’s cleaning ability is well documented. I just think one has to realistic about all the events that will happen after the switch. Frankly, I’d rather base line the engine, have it clean with excellent oil circulation, than wonder when a piece of sludge gets dislodged and causes an issue.

    I hope this helps.

  24. synthetic oils and lubricating fluids have a much smaller molecular structure than conventional oils. If seals and gaskets are marginal, synthetics will likely find a way thru them. Conventional motor oils of today are more than adequate to ensure long life service in everyday fleets. If it makes you feel better, and closer to your favorite F1 or Indy race team, by all means use synthetics. I do use synthetic exclusively in differentials and bearings/suspension/universal joints for the cold weather performance advantages. Colder climates are hard on machines. On the other end, machines love heat up to the max temp they’re designed to operate. I once used syn in auto trans and power steering but the run up in price the past the past 12 years has priced out any advantage IMO. Todays conventional products are well suited to the task. I also found that some private brand synthetics are becoming very competitive.

  25. Mobil has done plenty of reseach on this. Talk to their tech geeks. Syn oil is known to be a ‘leak-seeker’. Also, M1 sells a specially-formulated high-mi formula that contains some snake oils that work on the seals to probably soften and swell them.

  26. I’ve changed about 8 or 9 vehicles to Mobil-1 synthetic ranging from brand new (changed at 5k mi) to 250k mi. Everything from a Dodge Caravan, several Volvos both “white” and “red” blocks, to an MGB, to an old BMW motorcycle. No leaks that weren’t already there, and those did not get worse.

  27. I guess I’m old school, but I stick with what the manufacturer recommends. If it started with synthetic, keep it, otherwise why change it. It seems to me (and from the above lots of people will tell me I’m wrong) but a big benefit of synthetic is longer time between oil changes. I go the Mercedes recommended 10k between changes in my E320 and its fine. 1 Qt consumption in 10K miles. Engine has 120K miles. As for non-synthetic (can’t bear to use the cute Dino), I’ve taken a Mercedes to over 200K on it, my Caravan has 170K miles on 5K oil changes (have had it since new) and 1 qt between changes. I’ve taken most ever car I’ve had to over 160,000 miles with no issues. Stick with what the manufacturer says, they test these things, that’s my thought

  28. I’m a big fan of synthetic oil, but I have heard of leaks in older engines. My dad has some old motorcycles that he will not run synthetic oil in for that reason. The conventional oil is pretty good these days, but one of the advantages in synthetic is that it doesn’t break down over time, just from wear. Conventional oil will break down even if it is just sitting. For that reason I like using synthetic in my boat and vehicles that I do not use that often. When I was driving a lot of miles in my Honda Civic and changing my oil every few months, I ran conventional. I have a 2000 Ford Explorer that I bought new. The best, most reliable, cheapest to operate vehicle I’ve ever owned (better than the Honda). When I first got it I used conventional. Then Motorcraft came out with a synthetic blend that was about the same price as conventional oil so I switched to that. My last few oil changes have been with Mobil 1. I have over 215,000 miles. No leaks, no oil use between 5k changes, no problems.

  29. This argument has been going on for 20 years with the same points being made by both sides, and still no real conclusive empirical evidence. The only thing I can say for sure about the original article is that I’m pretty sure Fidel Castro had nothing to do with the oil you put in your car. Maybe you meant Castrol?

  30. About 10yrs ago I switched my new-to-me ’95 850 (w/ 66k miles), to M1. I think a year or so later I started having a RMS leak when I was trying the “German” Castrol 0w-30 synthetic. Was it the oil, deposits from old to the effects of swelling or not and associated wear of the seal?…but I certainly like the fact of having an internally clean engine. I’ve since replaced all but the RMS as I just don’t feel like pulling the transmission, nor the cost to have someone else do it for me (have heard $800-1K expense). I do have the seal and that for the trans input shaft sitting in my toolbox though. Not long after the leak started however I made a metal formed catch tray with an absorbant foam insert that I wring-out at each oil&filter change. The volume was never a lot, but regardless it’s been lessening with time as I’ve been using high mileage oil formulas, either semi or full synthetic. And yes, I’ve gone through and replaced vac lines and cleaned the oil separator.

    OT: The car’s been great otherwise, despite the usual issues with exhaust, brakes, and steel exposed to a salty winter environment. Kind of wish the heater put out a little more heat, but with watching the coolant temp gauge I can tell it’s giving all it can (wagon, a lot of 10 mile trips).

    Take care all…and focus on driving when at the wheel.

  31. This is oil mis-information HQ. It would be just as wise to conclude that it’s a Volvo thing rather than a synthetic thing. Even newer Volvo’s weren’t being shipped with Full Synthetic, I know this as fact for 2008 models. Honestly, the issue of ‘switching over’ is soon to be a thing of the past- since the early 2000’s many cars started shipping with Syn oil new. If you have a questionable service history on an older car and are unsure, you may be safer switching to Semi-Syn which will provide much of the benefits of full syn, but not shocking the system- that mainly being the rubber seals. Furthermore, not all Synthetic oils are created equal- do your research!

  32. What I see here is a bit of old wive’s tails, and partial information.

    Synthetic oils are excellent products, especially in the engines that were designed for them. Higher temps, turbos, higher, more efficient running temps and low friction rings benefit from synthetic oils.

    So MAY older engines, but the advantages of running synthetic oil in an 80s’ flat tappet engine are about the same as running premium fuel in a low compression regular fuel engine. Not much… Mostly, you are draining $$$ whenever you do an oil change, as you dump that expensive synthetic into the drain bucket.

    As for changing oil in an older car to synthetic, it is first important to see how it is doing on the old mineral oil. A cylinder leak down test is always a good idea, as is an inspection for leaks that would be common to the particular make and model, and a respect for the engine’s age and history. Do a conventional oil change, see how the engine performs.

    It the engine is running well, keep using the mineral oil, multi viscosity, as recommend by the manufacturer. Feel free to increase the top number, say, from 10w30, to a 10w40. This usually satisfies the need to improve engine protection in hot climates or during track sessions.

    Synthetic oil is of lower viscosity when cold, and will often find its way past worn seals where conventional oils would not. It will also find its way past old piston rings and valve guides. Note that not all seals leak externally. Having a good compression test with excessive oil consumption really means that the valve guides and seals are probably suspect. Note that older engines may have more “blow by” gasses, and that may contribute to the increased oil consumption as well.

    But worn seals are part of acquiring mileage, and if you have a nice fresh used car with more than 80K miles on it, having lived a life on conventional oil (assuming that was factory recommended), then continue on the conventional oil. Switching to synthetic wont protect the engine any better if all the rest of the systems are up to snuff (like the cooling system).

    My old Mercedes has lived a life entirely on Valvoline Racing 20w50, and it burns no oil at all. It has that built in Mercedes leak at the front of the cylinder head, exhaust side that it came with when new and was “fixed” 7 times. Enough already. I am not going to change to a synthetic oil, as it offers no advantages (and will only make this common oil leak worse). My engine runs fine, and is in excellent shape at 130K miles. I could have run Mobil 1 as I did in my racing Mustang, but I didn’t. Paying about $100 per oil change (cost of 9 quarts of synthetic and a filter) didn’t make any sense to me, and the oil change interval of 5000 miles that I was using seemed to be working out well. 20 years of driving has proven my assumptions correct.

    My race Mustang ran past 130K miles, with over 8000 track miles o the engine without problems related to lubrication. No leaks, no consumption (it remained consistent at a pint/1000 miles) throughout my 15 year relationship with the car). I started with the Mobil 1 from the 3rd oil change (I wanted to make sure that the engine broke in properly, so I waited for the oil consumption to stabilize prior to making the switch. This took about 2,500 miles).

    I did convert my Renault R5Turbo to synthetic about 20 years or so ago. It had around 20K miles at the time, and I wanted some extra protection for the old tech oil cooled turbo. I installed Mobil 1, ran if for only 1000 miles, and changed it again to make sure that I had only Mobil 1 15/50 racing in the system. The end result is no leakage at all, no change in oil consumption, and the turbo is still OK at 55K miles.

    But the difference here is that I owned this car since it was manufactured, and meticulously maintained it myself. I knew precisely the condition of every part, and knew exactly what I wanted from the change to synthetic oil. I made sure that when I made the change, that all of the old mineral oil was replaced by two transfusions of synthetic, as well as additional research into the possible wear on the flat tappets of the hemi head, pushrod engine. Valve adjustments have stayed precisely the same after 30 years of ownership, so I assume that the synthetic is working, and I have created no new problems.

    My recommendation is this: If you acquire a high mileage car, by all means service it, using what ever type of oil the car grew up with. If it is modern, and the recommendation (and current lube) was for synthetic oil, run that. If the oil is conventional, then don’t switch if the engine is in good condition, and running well. There is no reason to make a change. The engine will last just as long with regular conventional oil, with recommended change intervals as it would if you switched it to synthetic, except that it will cost a lot less to maintain on conventional oil.

    But just as you would not begin to run an old engine on 20W oil if 30W was recommended, note that if you change to synthetic, the thinner initial viscosity and flow will likely ooze past old seals and rings, and offer no real advantage in engine protection that proper maintenance doesn’t already offer, when using a high quality mineral oil of the recommended weight.

    You really need a good reason to change to synthetic. The resistance to “coking” in turbos is a good reason, or extra protection at high heat, high RPM. But if the engine already has miles on it, and the turbos are of the modern, water cooled type, and the recommended oils are conventional melted dinosaur, the stick with the conventional oil. Save your money for suspension or cooling system parts.

    If you have done an expensive overhaul on one of these older engines, break it in on conventional oil, to ensure that the rings seat properly (I can’t stress this enough. Wait until it stops using oil, or the consumption stabilizes), then switch to synthetic and enjoy the advantages.

  33. Similar experience here, but I choose to live and die by the synth sword. I changed the oil in my cherry 98 volvo v70 (120,000 mi) to synthetic and it immediately plugged the pcv/oil separator and blew the rear cam seals. Fixed those two things, stuck with synthetic, no probs. Previous car (2000 v70) started leaking out of the rms, when switched to synthetic at high miles but never leaked enough to make me fix it. All synthetic all the time for me.

  34. I put AMSOIL (5-40 Euro) in my 1996 850 Turbo 850 @ 96,000 miles. No Leaks. 1972 Mercedes 450SL @ 70,000 (10-40), stopped a small leak. 1990 Dodge Cummins @ 30,000(15-40) has never leaked.

  35. Sorry, Andrew, but your article was nothing more than the advancement of the classic fallacy of: “post hoc ergo propter hoc”– which translates into “If B follows A, then B is caused by A.” Just like stepping on a spider doesn’t cause it to rain, your article proved nothing related to your point. What it did suggest, however, is that you’re apt to buy high-mileage cars (likely on the cheap, especially since one wasn’t even running) and then are surprised when they leak. How you make the connection between your ‘issues’ and the switch over to synthetic lubricant, other than through anecdotal evidence, requires a leap of faith over an analytic chasm that’s much too wide for me to even consider following you.

    Oh, and for the record, you “itch” (not “inch”) for a Volvo 850. The sentence, as you wrote it, conveys exactly the opposite sentiment you were attempting to convey.


  36. I have a 91 940SE Volvo with 448,000 miles on it I have been using 10/40 Amsoil since I bought the car in 1994. I think it had 135,000 on it at that time. I have replaced the front and rear main seals probably 3 or 4 times. I normally replace the crank and cam seals when I change the timing belt. I also use Amsoil transmission and rear fluids as well.

  37. Ensure that the Castrol oil you can get is the same (spec) as what you can buy. The T5 Turbos require the proper spec. I don’t remember which off the top of my head, but ACEA A5 or B5 or something like that. If your Castrol doesn’t meet that spec, then you could void your warranty if Bad Stuff happens.

    Something I didn’t mention earlier was that my car was fine on 20K oil changes with the sporadic top-off vs. the factory recommended 10K changes. This was my 1996 965. Oil analysis including TBN with Blackstone revealed that the oil was still plenty good at that point.

    The economics matter a bit, but the more expensive synthetic means fewer changes in an engine in good condition. That means fewer changes and filters. Less time spent under the car or paying a shop.
    Of course, the oil analysis will set you back some, but that’s the price of knowledge. 😀

    I wager part of that cost could be put under “hobby” or “I’m a nerd” – the latter very close to “fun”.

  38. After working as a Tech for 30 years at a dealership the idea that Synthetic oil caused you issues is is there another word for stupid?

  39. Probably the worst article so far by this guy. After you get past the poor grammar and spelling. You can see that it’s all based on, well, nothing. All Volvos need a PCV/oil trap at about 100k, and a miata that has an obvious design flaw, is a ridiculous way to back up your claims.

    • ok
      I think most agree his article was a little bias or vented frustration.
      Let cut him some slack now and move on

      Lesson learned–is do not use personal experiences as a basis for an article to “educate” the masses

  40. Let me add to the hotly contested debate…..

    The ONLY true test is to run 50 cars on mineral oil and another 50 cars on synthetic then switch one group of 50 over to synthetic while keeping the other 50 on mineral. Then produce some data.

    Until an oil company produces these tests I’m going to continue to swap all my cars over to synthetic and enjoy the wear benefits. Cam seals leak and especially at high mileages (which is when they tend to fail)
    Secondly I understand that there is a higher detergent value in synthetic products but after having worked on engines for thirty years and seen first hand what sludge can do to engines, I’d rather have them slightly cleaner than having the sludge added to by mineral oils.

    Thirdly no matter how good a synthetic oil is, it can’t “clean” an engine that has significant sludge, causing oil leaks; only a hot tanking or manual scrubbing of the head/block can shift these nasty deposits. However it can maintain the status quo of the engine’s current state of cleanliness, which a mineral oil can’t do.

    • Mike, you are dead on: this page is full of anecdotes and logic, but not a lot of data.

      There is a lot of experience, and ideas on this here, but not a lot of science. Synthetic is different, in more ways than we might realize: are the molecules the same size/same shape? …is there something in standard/synthetic oil that is not in synthetic/standard that provides an unrealized function (like keeping seals saturated/pliable/stable)?

      The issue should not be dismissed, until the test is done. Until then, we really do not know.

      Everyone should keep in mind that it is human nature to look directly at the sign of a problem, and either ignore it or rationalize it away because it does not fit our established world view.

  41. Andrew, I also have an experience switching to synthetic after dino.

    Some years ago I bought a used but low hours John Deere tractor. It only had 1200 hours on it but like most equipment like this, few keep records of service. Right off the top, I opted to change the oil over from dino 10/30 for the Yanmar engine, to Mobil 1 10/30. The oil was disappearing every day but not from a leak. I called the previous owner and asked him if that was normal for this machine and the answer was no. So I pulled the plug and went back to Rotella 10/30 – problem solved!

  42. I purchased my 1971 Volvo 142s new and after the break-in oil, I switched to Amsoil 10-30 synthetic. I changed oil at 3k intervals, at 14k the camshaft failed..Volvo asked me what I had used to make it last that long… Slightly dumbfounded , I answered that I used Amsoil synthetic and their reply was that was why it didn’t fail at 8-12k. (Detroit mfg camshaft) If it failed under 12k they would warrantee it….so much for syn…Both of my Volvos, 2003 XC-70 130k and 2008 S-60 AWD 85k have full syn BG 5-30, all boxes and trans with no problems..Volvos 8 & 9

  43. After 4 headgasket leaks on Mobile 1; E320 (50K), Galant (99K), Integra (120K), Camry (225K) finally figure it out. This is over an 8 year period unfortunately. Recently tried Rotella T6 5W-40 and was impress with performance otherwise Swepco 306 15W-40 or 308 5W-40 for cold weather. Will rarely try other Dino’s. Been working on cars since the late 70’s.

  44. Howdy peeps, I have a 99 S70 GLT with about 180k miles on it, i had it for about a year and after o got it at the first oil change i switched to synthetic – i use the cheap “SuperTech” from walmart- and i change my oil and filter every about 5K miles; so far everything seems to be ok. no leaks or puddles.

    (i used to have a little problem with the car burning oil, untill i replaced the PCV breather system, but thats another story)

    what do you guys think of this “SuperTech” stuff.

  45. OK, so it has already been said, (correctly) that the Volvo issue was bound to happen no matter what because of lack ofmaintenance on the oil seperator/flame guard. This is because the oil seperator and flame gueard get gunked up from crankcase gasses which are laden with carbon and water and oil particles which coalesce to form the perfect cruddy-sludgy-crusty gunk that loves to clog up volvo flame traps and oil seperator boxes, which in turn builds up crankcase pressures to a point where crank and cam seals are pushed out of their recesses allowing oil to leak all over. THIS IS TRUE. So the Volvo example is not an example at all, agreed? GOOD.

    Now on to the second (non-) “example”: You really should make a thorough investigation as to WHY the synthetic oil was ‘vanishing’ and writing that as the reason you decide not to use synthetic. saying that the car has some sort of personal preference on the oil it gets is not anything that should be included in a tech related article. You should be finding out the actual specifics as to why the engine and oil are not compatible before using it as an example. That is unless you arent concerned with your own credibility as a automotive technical blog writer. Not to mention it makes FCPEuro look bad as well.

    In the future please do your homework before making recommendations to vehicle owners. Also, you should look into the details of the mx-5’s synthetic oil consumption issue and post a follow-up article so you can hold on to your reputation & credibility as an automobile-tech writer.

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