Do a search you'll find it.
Here's the quick short version:
-Sometimes cars which have over 100k miles and have not been fed Synthetic from the start, do not respond well to Synthetic. This is because, build up has occured by using regular oil throughout the years. Synthetic will penetrate deeper into the engine, it will also hit these areas where build-up is present. The build-up will then start to come off, possibly exposing cracks and such where oil will start to leak.