My wife has an '07 Acura TSX. The first oil change I did myself and her gas mileage dropped off a couple miles per gallon. The next one we took it to the dealer and her gas mileage came back. I just changed it again, and already she says it has dropped off a little. I couldn't imagine the dealer using synthetic, but I'm starting to wonder. Anyone else had Pennzoil lower the gas mileage a couple miles per gallon after using a different oil?