In addition to wheels, a vehicle’s drive axle includes a differential unit, which uses gears to synchronize the rate at which the wheels rotate. When your vehicle turns, the inside wheels don’t need to make as many rotations as the outside wheels, which have a greater distance to cover. The differential helps keep the axle from binding up from this difference in turning rates. A thin coat of oil lubricates these gears, and that oil needs to be changed from time to time. Your owner’s manual or maintenance minder will tell you when to replace it. If the oil becomes contaminated or the level drops too low, the gears could be damaged, which would lead to a more expensive repair or replacement.