Gear oil is used to fill a vehicle's differential. Ford uses a few different types of differentials, but the standard rear-end is a 9-inch or 8-inch differential. The fill plugs can be located on the rear cover or on the driver's side of the front of the differential, near the joint between the drive shaft and differential. Changing the fluid in the differential is similar for most types of vehicles and takes only about 30 minutes to complete. The main difference is the type and location of the fill plug.
Engage the emergency brake and turn off the vehicle. Elevate the vehicle. Place a jack beneath the passenger's side rear wheel and raise the wheel off of the ground. Place a jack stand beneath the axle. Repeat on the driver's side rear tire. Slide the oil bucket beneath the rear differential.
Loosen and remove the retaining bolts holding the differential cover in place with a socket wrench. Remove the cover.
Scrape off the gasket material from the differential housing and cover. Apply new RTV gasket material to the surface of the differential housing where the cover will sit. This is a good time to inspect the gears for signs of wear and tear.
Install the differential cover. Hand-tighten the bolts down in a criss-cross pattern starting with the lowest bolt, going to the highest bolt second and continuing around the circumference of the cover in the criss-cross pattern. Tighten with a socket wrench once the cover is fully seated.
Remove the fill plug with the socket wrench with no attachments. The head of the wrench will fit into the fill plug.
Insert one end of the tubing for the hand siphon into the differential and the other end into the bottle of SAE 75w90 gear oil. Fill the differential until a small bead of oil drips out of the fill plug hole. Reinstall the fill plug and tighten it with the wrench.
Lower the vehicle one wheel at a time, removing the jack stands as you do. Monitor the differential for signs of leaks after driving the vehicle for some time. Repeat Steps 3 through 7 to correct the application of the gasket material, if needed.
