Rear and front axle housings, which contain the axles and differential gears, also require fluid.
Types of Differential Fluid. Gear oil (e.g., SAE 75W-90, 75W-140, 80W-90, 80W-140, 85W-140) is the most common differential fluid. Some differentials use limited slip additives. Always check the owner’s manual for the specific fluid.
Checking/Adding Differential Fluid. This fluid check is performed on rear-wheel drive (RWD) and four-wheel drive (4WD) vehicles. The process is similar to checking manual transmission fluid. Remove the check plug, check the level, and fill as necessary. On most vehicles the oil should be at the bottom of the plug hole, but check the owner’s manual for the required level.