Its my understanding that when a car is put in neutral going down a hill the tranny is not getting the proper amount of lubrication to some parts. Obviously it has its own oil so maybe its something to do with the diff in engine vs trans speed. At any rate if this is true then any money saved by putting it in neutral can be used for a new trans later on. But i cant imagine how much hes trying to save unless these are some extremely long hills. And if hes that worried about saving a few pennies maybe he should buy a Dae-woo not a Mercedes-Benz
