Go by what the manual says. The retard at the dealership is exactly that. Remember that if you are here on this forum and interested enough in your car to participate here, then you most likely know more about your car and what it needs than any dealer or salesman. Just because somebody works at a car dealership doesn't mean he knows squat about your car or how to really take care of it. The service folks maybe, but they have oil changes to sell so you can't trust their judgement inherently either. There's so much folklore and such out there about people making engines last forever by following ridiculously low oil change intervals and such... but really, the manufacturer has told you exactly how to do it so that's the way to go if you ask me.