The Ford 9-inch rear end is one of the most commonly used rear ends in high performance vehicles such as the Ford Mustang GT due to its bulletproof performance. In order to keep your Ford 9-inch rear end working in top condition you must keep it properly oiled with gear oil. The Ford 9-inch rear end uses the fill hole as the oil level indicator and it does not have a dipstick. Because of this, you must keep the Ford on level ground while filling the rear end. This will ensure you do not put too much or too little oil into it.
Crawl under the rear of the vehicle and locate the rear differential housing. The housing is the large "pumpkin" located in the center of the axle.
Locate the plug with the square hole. You should be able to find it on the left backside, towards the top of the differential housing.
Remove the plug by placing a 3/8-inch ratchet driver into the square hole. Turn the plug counterclockwise to remove it.
Place a drain pan under the differential to catch any spilled oil.
Cut the tip off the oil container using a razor blade and position the container upside down with the tip located inside the hole in the differential. Depending on the model of vehicle, you may have to hold the oil container at an angle and squeeze the container to force the oil into the differential. Cut a slit in the bottom of the container to help the oil flow quicker.
Remove the container from the differential when oil starts to seep out of the hole. Allow all excess oil to drain from the differential. Usually 5 minutes is a good amount of time to allow excess oil to drain.
Thread the filler plug back into the hole using your fingers to avoid cross threading the plug. Tighten the plug using the 3/8-inch ratchet driver.
Wipe the excess oil off the differential using a shop towel.
