Does full synthetic really cause leaks?

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Originally Posted By: WobblyElvis
I have a 99 Dodge Caravan that used conventional oil most of it's life. I put some synthetic oil in it 2 years ago and it had a small oil leak from the head gasket at the time. The leak became very large and I have not been able to stop it from leaking ever since. "Stop leak" additives to not work. There is a tray under it in my driveway right now. I will never change from conventional to synthetic ever again in an older vehicle.
You're leaking oil from your head gasket? Sure its not the Valve cover gaskets? Thats not THAT hard to do on the motor. Try Valvoline MaxLife for several changes, I have had decent luck slwing down leaks with it.
 
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Originally Posted By: wemay
Cause? No. Exacerbate an already existing leak on a car previously using conventional oil exclusively? Maybe. But I also believe that an oci neglected vehicle, switching from a cheap conventional with a poor additives pkg to one of the major brands using correct oci, could do the same.
Exactly. Synthetic oil will not cause a leak however if there is a defective gasket it may expose the defect. In every engine I own I use liqui-moly motor oil saver. It's an ester and reconditions gaskets to prevent possible gasket leaks. I've got trucks with 300k,work vans with over 450k and not a single one leaks oil.
 
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When I switched to Mobile 1 10w40 in my '80 Yamaha it developed (in one way or another) a couple of pretty bad oil leaks. Even the brand new valve cover gasket weeped a slight bit. After replacing a bad clutch shaft seal and switching back to dino oil it doesn't leak a drop. Switched to Napa full synthetic in a Chrystler Sebring at approx. 95,000mi and it did not develop any leaks. Switched form MC blend to QSUD in my F150 at 48,000 miles without leaks.
 
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Originally Posted By: jayg
[You're leaking oil from your head gasket? Sure its not the Valve cover gaskets? Thats not THAT hard to do on the motor.
Motorcycles often use one or more of the cylinder head studs as an oil passageway, with an O ring around the stud at the head. They often leak there, synthetic oil or not. Z
 
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My 99 van is definitely leaking at the head gasket, not the valve cover. It was leaking slightly before the switch to synthetic. No stop leak type of additive has made any difference. Switching back to conventional does nothing. It's no big deal, I'm just sharing my experience so that others can make a more informed decision. I know I will never try a synthetic oil in an older engine again. For what little advantage there is to synthetic oil, it was not worth it for me.
 
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So, what is "synthetic" really? Most of today's so-called conventional oils are Group II+. Would you consider Group II+ conventional in the 1980s? Do you think Group II+ is not bad on seals if no seal swellers are used in the oil? So, the whole question of whether synthetic is bad on seals it's not even well-defined. Are we talking about PAO or Group III? Why are we ignoring that Group II+ is not that different than Group III. See the following article. Group II+ oil (conventional oil) is as bad on seals as so-called synthetic oil unless seal swellers are used in the oil. Again, oils and seal compatibility should be your least concern. All oils have enough seal swellers to maintain compability. Of course, too much seal swellers will also harm the seals; so, it needs to be within the API or ACEA performance-test specs.
 
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