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 have a 1959 John Deere 2 cylinder gas bulldozer that I switched to mobil 1 +/- 35 years ago over the objections of several “experts” who predicted an early demise for the engine. In the thousands of hours since I still await its failure. It uses very little oil and starts willingly at o degrees.( It”s 6 volts!!.) I switched an early 1970″s diesel powered forklift to Mobil 1 twenty years ago with the same results. I have an Isuzu industrial stationary generator with over 14,000 hours that has had Mobil 1 Delvac since new and shows no signs of wear. I have 4 Volvos ranging from 1990 to 1998 all were purchased used and all were switched to Mobil 1 when I bought them. With the exception of the 940 which had a bad turbo when I bought it all have the original turbos. One of these cars has 80,000 miles and the other three are over 225,000 miles. I think that synthetic oils are particularly important for turbocharged cars because when they are switched off the turbo does a “hot soak” which can heat the small quantity of oil that is in the turbo to a point where conventional oils lose their viscosity. The synthetics can be heated to a much higher point without that happening. In my experience everything that conventional oils do synthetics do better.

    • I have been in the auto and truck repair business for over 30 years and have not experienced any negative effects after switching older engines to Mobil 1. Most of my own vehicles that I have purchased used with high mileage both gas and diesel. My most recent vehicles were 200 and 2002 Ford F250 and Ford Excursions with the famous 7.3 diesel V-8. The Excursion had 293,000 miles from Texas and the F250 had 149,000 out of Florida. I cant be sure of what oil they were used to but I can say that I switched them right away to Mobil 1 5W40 Turbo Diesel oil and have not had a problem. Just my 2 cents. I have heard of what I considered a myth about changing to synthetic but I figures it was similar to what they first said about radial tires not being able to rotate them from side to side ? Just my 2 cents. Thanks

    • I’ve used synthetics in all my cars, trucks, and heavy equipment, both gas and diesel. My 65 Bronco with 200ci six, Z28, Trans Am, Silverado, Ranger, BMW 325is, Cat and Detroit diesel, snowmobiles, ATVs, and air tools.
      I have never had your experience in any used vehicle. I am quite sure your seals were damaged and leaking before you put synthetic oil in, if you go to conventional, especially one with low detergent your engine will produce enough dirt to plug the leak again. The inside of your engine will look like a sludge heap but it won’t leak.
      Regular oil changes with cheap conventional oil wi produce a load of crud which is probably what damaged those seals initially.
      Oh and even the lawn mower has synthetic, as well as all trans and diffs. I use synthetic grease only as well.
      I’ve been using synthetics since the 70s for cold weather use in our log hauling operations here in Manitoba, Canada.

  2. My general approach is to switch to synthetic and monitor the vehicle’s signs like starting in cold, leaking, oil consumption, etc. Never have had any engine failures, yet. However, one vehicle purchased 2 years old, when switched to synthetic, consumed 2 qts of oil in 5,000 miles. It went back to conventional oil and 3,000 mile intervals changes with minimal oil consumption.

  3. There is no possible excuse for NOT running synthetic in a Turbo engine. The reason is the heat generated in the turbo cooks petroleum based oils and they break down while synthetics will not.
    I am a firm believer in synthetic oil and have been since the early 1980’s when I first used a synthetic in a 600+ HP SD 455 Pontiac engine that was built for a 1977 Trans Am I owned at the time.
    After 25000 punishing street miles and many many passes down the strip wit was pulled down for some minor mods. What we found made me a permanent believer in synthetic as there was not a single trace of sludge nor any measurable wear to be found in the bearings, bore or any other area of that engine. It was shifted a 7K regularly and was geared in the end at 2:41 with the intent of running the Motor Trend Cannonball. Top end was just under 200MPH and capable or cruising at 150 for as long as there was fuel to do so.
    We ran the new at the time Penzoil full syntheticin that one and since we pulled that one down I have never run petroleum oil again in anything.
    I have never had any issue with synthetics other than a slight increase in consumption when switching to it in high mileage engines.

    • Richard, my 91 Volvo 940 Turbo has 440,000 miles on it. Since 94 when I bought the car I have used Amsoil in the motor, trans, rear and the P.S.. The car runs and looks today just like it did in 1994. No major replacement parts only an occasionial timing belt and a few seals. For me it is just good economics. With Amsoil this story is not rare but normal.

  4. In my experience, what makes synthetic oil so great is what makes it problematic in ANY motor that is not (extremely) well-sealed. If it leaks with dino, it will leak more with synthetic. If a seal is borderline, it will show with synthetic. The “problem” isn’t the synthetic, it’s the failed or failing seal. The leaking synthetic is only the symptom.

  5. I owned a 93 toyota pickup 4cylinder. Put 456,000km’s on it. Switched from dino oil to whatever synthetic was on sale in the $20’s around 360,000km’s. The first head gasket blew just before the switch. Ran another 100,000km’s on synthetic. Engine ate oil after around the first 125,000km’s because I drove the engine really hard and was a bit late for the odd oil change but basically put new dino or synthetic oil in every 5000km’s. The engine only leaked because whatever seals or O-rings or gaskets eventually denature and become hard. The rubber or silicon seals do not last forever regardless of what type of oil. It’s the same as the tires. If you leave a brand new tire on the shelf for five years, that tire is no good despite the full tread depth. It starts to become hard and loses its elasticity and will break. Don’t expect your seals to last forever on your cars. It’s the same reason why shocks need to be replaced. The seals don’t last from all the wear and tear. Radiator hoses become hard and brittle until they fail. Anything that is rubber or elastic in your vehicle will not last forever and needs to be replaced with time and age. Otherwise it gets old just like you will. lol.

  6. In ref to the author’s long article – mere anecdotes relating coincental events. No causation is proven between switching to synthetic oil and minor problems developed thereafter.

  7. The belief that switching older cars from Dino based oil to a state of the art synthetic oil will cause leaks and is detrimental to the oily bits is utter non-sense. The root cause of why and where the oil leaks comes from needs to be understood.

    Over time and service life of polymers used in oil seals DO NOT LAST FOREVER. The most common seal material used is Nitrile rubber, aka Buna-N, aka NBR. The stuff takes in lubricants over time and hardens loosing it’s elasticity and ability to create a seal. If changing over from a Dino based to a modern state of the art synthetic results in oil leaks, those tired old seals are well ready to be replaced anyway. The change in lubricant environment -might- speed up the eventual oil leak, but the synthetic oil itself is NOT the root cause of the problem, wore out an tired oil seals are.

    We have been using state of the art synthetic oils in everything from a 1957 Triumph TR-3 to modern cars built in the 2000’s has not resulted in oil leaks. The benefits of state of the art synthetic oils can be validated by oil testing and careful measurements of engine, gear box and other oily bits when the mechanical assembly has been taken apart for service or rebuild.

    It is rather irresponsible and media malpractice to continue to perpetuate the urban myth of modern, state of the art synthetic oils a the root cause of oil leaks is pure folly.

    Do the work, study and fact based research before publishing what has been proven to be a urban myth.

  8. I have always run dino oil and switched it + filter regularly at 3K miles. My vehicles have always been runnign strong at sale, etc. with over 250-300 miles on them. I’ve never experienced an oil related failure. I can’t justify the increased cost of syn oil but, I don’t run turbos – maybe that would be justification as one commentor stated. Additionally, there are acids which get into the oil from combustion and I don’t like leaving that in for the perscribed length of oil changes by OEMs WRT syn oil. I switch my new vehicles to dino.

  9. If I own it and it requires oil, it has Mobil1 in it. I agree with Mr. Mote. Synthetic oil is the symptom, NOT the problem. The molecules in Mobil1 are much smaller and will “slip” past any weak seal. I don’t have a problem with that because that issue is far and away offset by the benefits. It would be TOTALLY irresponsible to put non synthetic oil in a turbo charged engine. Mobil1 is in all of my lawn equipment, my air compressor, and of course all of my cars. Everything runs cooler and more efficient which translates into improved fuel economy.

  10. I always buy 100k + cars, do the first oil change with synthetic blend as a transition, then do the next oil change with full synthetic. Never had a problem. Bought Volvo 855R with 183k, switched, drove perfectly and sold at 208k. Bought Saturn SL with 33k, switched to synthetic around 90k, ran excellent when sold 10 years later at 160k. Bought Honda Pilot at 95k, made the transition, currently has 167k, no problems whatsover. Bought Dodge Ram 4.7 at 186k, switched, currently driving with 206k and no problems. And I’m not a Mobil 1 die hard either. I just use Walmart syn.

  11. I had the same problem as the author. Volvo 850 with 100k. Put synthetic oil in it and the rear main seal started leaking. But I didn’t replace the seal, I just switched back to conventional oil. It took awhile and a couple of oil changes but the synthetic got flushed out and the leak stopped. And for those of you who insist the real problem was a bad seal — you are wrong. The engine now has 200k on it and that rear main seal is still not leaking.

    • Nice job Bob. It seems some people will argue the fact that the sky is blue just because they can. The author is simply stating his experience & belief in synthetic oil. It’s pretty simple,
      petroleum breaks down much much quicker than synthetic

  12. I picked up an S70 Volvo some years back that had about 100K miles on it and after a few months of driving switched to fully synthetic oil. After a few weeks it started blow smoke when hot and after idling for a few minutes – sounds like valve wear right? Seemed to me that the synthetic oil (which has detergents) ‘cleaned out the engine” e.g. carbon build up and possibly opened up the valve clearances creating the smoke blowing problem. Under load the car didn’t blow smoke at all.

    Perhaps if the car has had regular oil changes on mineral oil then the switch to synthetic is not a problem?

  13. Back in the mid 70s, I worked at a mercedes, porshe audi vw dealer.we switched from valvoline to castrol, WOW, what a difference.Both dino oils, yet all brands would would turn inside of engines redish and even with religous 3,000 mile change intervals, they tended to build crud, EXCEPT CASTROL..We actually had numerous filters clog, hence,low oil pressure issues, due to cleaning performance of castrol.we started advising new clients to change thier filter at 1,500 miles.
    I run mobile 1 in all my vehicles and implements, from my allis chalmers diesel to my wood splitter.I am a retired factory Mercedes tech of 40 years and never bought into the 3,000 mile interval.oil change.Only if somebody does extremely short distance drives, where engine regularly dosnt reach operating temperature.My 98 E320 is what Mercedes calls a flex service vehicle, All 98 to around 2006 Are flexcars and fitted with a oil quality sensor.Indicator tells you when its needed.My car usually requires changed between 18,000 and 21,000 miles.I have 204,000 miles on it with no issues.The ONLY problem with this system is the fact people buy cheap oil filters designed for 3,000 miles.

    • Dwight: “The ONLY problem with this system is the fact people buy cheap oil filters designed for 3,000 miles.”

      What’s the ‘feature’ of a filter that makes it cheap? No drain back valve? What filters do you prefer, and why?

  14. i buy whatever synthetic is on sale at CTC or Walmart in the fall. Cars start much better in the extreme cold with synthetic. I switch back to Rotella in the spring when the 19l pail goes on sale. It goes in my two bikes, and the two cars. Never had any issues switching back and forth. If it leaks, it needs fixing.

  15. In response To Robdiesel, I Am also A Mercedes Tech. MB Factory Oil Filters are Made Of A Fleece element And fit The housing Much More Securely than Aftermarket Versions. They Last Longer And Filter better, I Suspect They Flow Better Too, But I Can’t Say For Certain.

    • OEM filters for MB (and Volvo/BMW/VW I believe) is Mann/Hummel. They also make the Purolator filters, so the argument for OEM would be that they are made to tighter tolerances?

      Would this affect more than the seal between filter and block?

      It would be nice (now that I think about it) to see some sort of filtration study. “After x gallons of oil going through, it still filters down to x micron” or something.

  16. I have a 1999 VW Passat 1.8T that originally used dino oil. The oil passages in these engines tended to build up sludge over an extended period that could cause catastrophic engine damage. There is an ongoing class action lawsuit to this effect.

    I participate in an online forum (passatworld.com) where I have been able to learn about this and other ongoing problems with this model such that I have been able to hed off any problems before they materialize. The oil sludge fix required that I run Auto-Rx sludge remover through my car before switching over to synthetic. You add it to regular oil when changing the oil and then run it for about 1500 miles. Repeat the process one more time and then swithc over to synthetic. I haven’t had a single issue following this protocol.

    On another note, I tried switching a ’75 Toyoto Celica over to Mobil 1 when it was first introduced. I found that it would go through a quart a week. At about $5 a quart back in those days, it got real expensive pretty quick. I switched back to regular oil and never had another problem with excess oil use or leakage. Go figure.

  17. I had my 2012 Yaris in for an oil change at the Toyota dealership yesterday. The service tech wanted to verify that I wanted conventional. (They have always put conventional in.) I asked him what the service life was for the synthetic oil and he told me it was the same and he recommended going with the conventional because that was all that the service manual called for. For the first time I also paid attention to the weight of the oil. 5W20. Both kinda surprised me,

  18. I have BMW 520i 2002 it’s 175k my rear main seal wasn’t leaking previous user was using 20-50 Dino oil and we are living in hot area then I switched to 5-30 synthetic it start leaking I’ve added a bottle of bars rear main seal but still the leak is dripping bu it covered the rear main seal bell and the oil pan but no noticible lose in oil until I pass 3000-4000k I have to add 500ml of oil
    But is there a way that stop that leak or shall I go back to Dino oil ??
    Or use autorx stop leak additive ??
    Any help??
    Or I can switch to synthetic 10-60 ??

    • I had the same problem with my Volvo 850. I switched back to Dino and the leak was fixed in about 1000 miles. Whether Dino or Synth I use 5-30 wt oil. I am also a believer in those High Mileage oils with their seal conditioners.
      Oh, I forgot, we’re not supposed to call it Dino oil. It’s too cute for some peoples refined sensibilities……………….
      Keep Smilin’!

  19. Thanks dick you mean that I have to switch back to 20-50w oil so may be my leak will stop
    But really i don’t like Dino oil I can run it only on Toyota tercel,corola,yaris,Eco
    But bmw recommend 10-40 but I’ll try for few thousands hope it’ll stop leak
    Thanks for your advice 🙂

    • My thoughts are-
      Go back to Dino but use the mfr recommemded 10w-40. Use a High Mileage formulation.. If it does work to stop your leak, it will probably happen in 1 or 2 oil changes.
      Good Luck!

  20. I mean dino=conventional oil
    I’ve used semisynthetic oil more than 2 times high milage but it’s not working shifted then to full synthetic hope it’ll work for me but it did nothing
    Any way I’ll try conventional oil with hope it’ll slow it down or stop it

    • I would recommend a rear main seal replacement. Oil is routine maintenance and should never be considered as part of a repair. Oil lubricates your motor, seals hold fluids in. Don’t confuse the 2

    • When it fails completely, so will your motor. Is your motor worth more than $20? If you dont’ want to spend $1000, learn to do it yourself. That’s what I did. Good luck with that car!

    • That’s the whole problem. In an older high mileage vehicle it’s just not wise to start pouring serious $$$ into it. Of course, Calvin is correct, of course, it’s just not practical. You put $1000 in now, then when the tranny goes out in a few months you think “well, I just replaced the RMS, probably should do the tranny” until you are way too deep in a vehicle worth very little. Cars don’t “wear out” like they used to. They just get to the point where they need repairs approximating 1/2 of their value. Then they become parts cars or just limp along until a catastrophic failure happens. That’s where my old 850 is right now. I will miss her terribly when she goes………..sniff. But not enough to spend a grand on a RMS, another grand on the AC Evap, the rear seat is looking a little sad, etc. Still supremely comfortable, runs like bat out of hell, and I LOVE the 5 Cylinder Warble!

      • Exactly. And that is what is so funny about these threads: there is a cost-benefit analysis to be made. I guess some people are expecting to keep their cars forever, in which case a $1k repair is worth it; I’m expecting another 2-4 years from mine (although it’s not my main vehicle).

        My experience mirrors your experience as well: I switched to M1 in a 200k mile Volvo and my seal started leaking. This was after a similar experience with a 120k mile volvo where the same occurred. In both cars I switched back to dino and have had the leaks mostly stop, although I did have to add some Blue Devil in one car. If I keep the car another 10-20k miles I’ll be quite happy.

  21. I’am checking under my car every day and oil level twice a day so Iam aware about that
    Thanks for your advice 🙂

  22. Nice so I’ll switch to 10-40 or 20-50 conventional oil and hope leak stop and will continue on that oil if it continue leaking and getting worse will do the seal as I’ve changed most of the suspension rods pcv valve and hoses renew cooling system and hoses
    New alternator ,new front and tail light
    New tires ,retrofit front and rear parking sensor need also to repaint it the offer painting in my country @about 500$
    Worth it
    I’ll keep it for 5-7 years
    It’s my daily driving can’t stop it only if I took aleave for few days in the future if that didn’t solve problem
    And nice driving 🙂 make you forgot all what you have paid 🙂

  23. Yes, once you start to spend real money on a car, it makes more sense to keep it running. Just curious, where are you living? Is it expensive to maintain a car there?

  24. I’am a Diyer
    See how it done by some people on YouTube and forums and doing it my self if it cheap labour I took it to the mechanic and he is doing the job
    But if you compare spare part for German cars and Japanese cars Japanese spares is more expensive but there labor charge is nothing compared to German cars
    Here in our country they see if you own European cars like merceds bmw vw Opel also range rover jeep etc you are rich man and if we do changing oil labor charge for Japanese car @ 10$ we have to take 40$ from you that’s the way except some good mechanics they cooperate with you and give you reasonable price
    Thanks god for that
    And you know if you drive bmw you can’t drive another car and if you drive another Japanese car you’ll return back like me owners of e36 ,e46 then Nissan altima then e39 now only got this e39 I love it mor than my wife 🙂

    • The 2002 V70 base I bought (w/80K) was maintained well after warranty but I do believe the owner used dino as there were deposits on the dipstick and “Quick Lube” receipts in the glove box. Steel wool, denatured alcohol and elbow grease took it off the dipstick.

      I went with dino in the warm months and synthetic in the cold for 5 years so not to shock the system.
      Now I’m going with basic Mobil1 5W-30 all the time.

      At 173K I’m beginning to think it might be time to change my oil trap box and related tubing. What does anybody think? I think there’s a flapper or diaphragm within the box which ages separate from any accumulations of sludge.

      • 173K on regular oil, yeah, you probably need a PCV replacement. with all hoses. I’ve always changed right over to Mobile 1 synthetic. If it leaks, it has a leak and I fix it.

        The whole operation took me two afternoons and some swearing. The banjo bolts are murder.

  25. Hello there, just became alert to your blog through Google, and found
    that it is truly informative. I am gonna watch out for brussels.
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  26. Hello just got leak I think it was the rear main seal
    It was the oil pan gasket in my bmw 530 model 2002 it’s pita to change and took it to my Indy garage he told me we have to remove the bracket ,rack steering and all arms to reach the oil pan it will coast you minimum 1k hahaha
    And I did it cause I love the car and will keep it for another 5-7 years

  27. But ask a bmw specialist he said just loosen the engine mount and raise the engine 5cm and you’ll be ok to change and when I told that shop he said if you can do it in this way I’ll raise the engine come and do it for free
    I said ok do it as you want and he charged 1200$ plus parts and oil change

  28. I agree with the original poster. I’ve got a 2004 Volvo XC70 with 137k. Paid for a synthetic blend oil change and several months later, it’s got a slow leak. Could it be coincidence? Of course. But there are lots of reports of cars that switched from conventional suddenly losing oil. I’m sure the gaskets are losing their seal, and the old oil probably self sealed. The synthetic flushed out the gunk and proceeded to start leaking. I’ve gone back to conventional. If I can get another year or so out of the car without a costly repair, I’m ahead of the game. We’ll see how it goes….

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