> I've heard that it's not a good idea to change to synthetic oil in a > high mileage engine. > Any thoughts on this? Well, if your gaskets are old and only holding oil in by virtue of waxy deposits formed from the dissolved hydrocarbons present in dino oil, then yeah, best not to go synthetic. Why? Not sure. I've heard two competing theories: 1) Synthetic will tend to dissolve those waxy deposits which were actually acting as a type of stop-leak sealant. If the gaskets are new enough, they'll eventually become saturated with and softened by the synthetic oil and be "rejuvenated", sealing the leaks. Gaskets thhat are more waxy deposit than gasket material, however, will just leak. This is the more plausible explaination, I think. 2) Synthetic is "slipperier" than petroleum oils, and thus more easily "slips by" the looser tolerances on an older engine, thereby leaking out. This one simply makes no sense to me, as I was under the impression that "slipperiness" was another word for "viscosity", which varies depending on the grade, not its synthetic or natural origin; but Autozone put it on their web site, so I thought I'd repeat it for amusement's sake. I ran synthetic in my 167Kmile bottom end (80Kmile on new heads since '97) and never saw a single leak. Of course, I only got about 2200 miles out of it before it croaked of a broken head stud, but there sure were no oil leaks. |
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