I've got about 120,000 on my engine...4th gen Accord. I was using Castrol 5w-30 dino oil until this last oil change. I talked to a good friend who has switched a few different cars from dino to synthetic and he said that it shouldn't be a problem. He said that the way that people start getting leaks is by going back-and-forth between synthetic and dyno. It expands and contracts the seals all funny-like and causes them to get screwed up (don't you love the technical terms? ) He said as long as I switched to a higher weight (Havoline 10W-30 Synthetic now), there shouldn't be any troubles. Since I switched, I noticed the engine feels alot smoother when it's running...it never really felt like that with the dino oil.

Just my 2 cents worth...