I'm with you there. I don't see much of a difference in carbon build up when I tear down motors, but the smell of the non synthetic stuff really sucks. I've gone to using a fairly expensive synthetic with my little boat, that actually smells good. Sitting next to a stinky engine all day isn't any fun.
I'm too cheap to run it on the big boat, though, and it really doesn't smoke too bad when warmed up or underway.