Synthetic oil acts as a detergent cleaning out all the gunk out of the engine including all the gunk that accumulates around the 10 year old seals preventing them from leaking. This is the reason why dealers do not recommend switching old cars to synthetic oil, since the new thinner oil may start all sorts of oil leaks.
Originally Posted by ScotcH
I don't know if I buy this. Why why would synthetic be bad for seals? Certainly it can clean your engine, and it will make you burn oil if it was very dirty, but actually causing damage? I dunno .... Mines not <92, so it does not affect me, but it's still an interresting post. I'd like more info (and maybe proof)!