There are synthetic fluids in the truck from new. It won't hurt to change them though. Everyone always forgets the biggest thing in fuel milage, the guy running the pedals. How you drive is the biggest factor. Not saying you don't drive well just something to reevaluate. Go to thedieselgarage.com to the heavy duty forums and ask what works there. w900s with under cab muffler are restrictive. If you're in the trucking business fuels the biggest expense. So it means either profit or peanuts.