One mechanic (or auto service employee) told me he doesn't use synthetic oil because it doesn't contain as many detergents as conventional oil. As a result, your engine becomes dirtier.
He even said the reason synthetic oil does not need to be changed as often as conventional is because synth oil does not get as dirty since it doesn't have as many detergents to clean the engine!
Another mechanic (or auto service employee) I spoke with had no knowledge of the first employee's claims. It seems if you talk to 4 people, you'll get 4 different answers ;-)