Synthetic Switch on a 5 Year Old Car

[chevy4x4trucks]

My cousin has a 96 Bonneville SSE w/ a 3.8 V6 and has about 70,000 miles on it.  The car has been taken in to quick lubes for oil changes it's whole life and has been running regular 10w-30.  Despite my warnings he decided to switch to synthetic about three days ago.  I had heard that w/ an older car switching to thinner oil would be a bad thing, leaking out the seals and such.  Maybe I was given some bad info, but I thought that was a no-no for engines.  So some of you are more knowledgable than me, what do you think???

 

Also, we wer in parking lot and there was a big pool of oil under his car that when I touched was almost clear (like synthetic) and was still very wet.  He says that wasn't his.

[Shaners]

I'd never switch a car over after 70K.  Actually the standard motor oil cause gaskets to swell more than synthetic and to stay swollen.  As the gaskets start to shrink a little the synthetic can seep by...

 

Personally I think he would have been better off staying with pretroleum oil and continue with regular oil changes.  I really don't see much advantage in engine life after 70K by changing over to synthetic...