F-150 Rear Differential Oil Change
This procedure was performed on a 1998 Ford 
Follow these instructions at your own risk.
Tools/Parts needed for the job:
- Safety glasses
- Rubber gloves
- 3/8 Ratchet, and 1/2 socket
- Brake Cleaner such as Brakleen
- Drain pan
- Small funnel with hose
- Rags/paper towels
- 3 quarts Redline 100% Synthetic 75W90** gear oil (Ford specifies only synthetic!)
- Replacement differential cover gasket
- Gasket type blue silicone sealer
- scraper blade or flat wide chisel
- flat blade screwdriver
Time to do job:
- Less than 1 hour
- Wait until the vehicle has cooled off for at least 1/2hr if you've just driven it to avoid getting a "hot oil treatment"... burns hurt!
- Position the drain pan under the rear diff cover.
- Remove or loosen all the cover bolts using the 1/2 inch socket ratchet. Note the position of the two tags on the bolts.
- Using a large flat blade screwdriver pry the bottom of the cover up slowly. When most of the fluid has run out pry the cover off the axle. The old fluid was really black and dirty.
- Use a solvent or brakekleen to clean the diff cover in combination with rags or paper towel.
- Use a flat edge scraper or chisel to remove any old excess silicone sealer. My factory sealed diff didn't even use a real gasket, only black silicone sealer.
- When the cover is clean move to the axle and using your finger or a spatula and remove as much old oil as possible. There is a small pocket in the diff that holds half a cup of oil in the bottom. Try to slosh it out or use a few paper towels to absorb it. Wipe off oil anywhere you can get to on the gears and walls inside the axle.
- Now clean the gasket surface on the axle with a scraper and some steel wool to get it clean. Finish off with solvent on a rag or paper towel to remove any trace of grease/oil on the gasket surface.
- Clean the diff cover in the same manner.
- When the surfaces have dried, apply a bead of silicone gasket to the cover and to the axle gasket surface.
- Apply the new gasket and press it down gently to spread the silicone slightly.
- Wait 10-15 minutes for the silicone to skin over.
- Apply the cover to the diff, insert all the bolts and tighten them lightly. There is no specific order. I started at the bottom, moved up the sides, and finished at the top bolt. I believe the Haynes manual states 30-38ft/lbs. Tighten the bolts all one more time to spec, then check each one for tightness.
- Insert a 3/8 ratchet into the drain nut and remove it.
Contributor's note: The information found here to replace my differential seal (and change the fluid) was great--with pictures even, but the contributer missed one cruciel bit of information to complete the job: the filler plug requires about a 3/8 square driver--not a common tool, and not mentioned. Mine was locked tight (I tried WD-40 and using a 3/8 socket end, but it wanted to strip), so I ended up just securing the lower end of the cover plate and filling from the top. It's buttoned up, working fine, and pretty now, but that square plug really threw me just when I thought I was done.
- Clean any metal shavings & sludge of the plug.
- Insert a small funnel and tube into the hole or use a transfer pump (easier, less messy).
- Fill the case with just under **3 Quarts (5.7 pints spec) for F150 and Expy rear axles, F250 are more. Or fill the axle on level ground until oil starts to run out of the filler hole. I call gear oil "yummy lube" because it stinks so bad.
- Replace the drain plug. Loktite is optional on the threads of the plug.
- Wipe any spilled oil.
- Clean up.
Installing a replacement gasket is also optional. Some prefer blue silicone only but it needs to be tacky (>15mins wait) before putting parts together or the silicone will squeeze out from between parts resulting in leaks.
