I am getting to the point where I want to do the first oil change for my drive. The manual says to use 10W40 motor oil. The dealer told me not to use synthetic oil like Mobil1 and that I must use plain oil and this additive from Hustler. I have always used synthetic oil in transmissions and other hydro drives. It tends to make the unit run cooler and quieter. What should I do since I don't want to void the warranty or anything.