Amsoil gear oil

There have been numerous discussions at about the worthlessness of the Ford limited-slip differential. I am one of them. I have had my truck for 2 1/2 years now, and I have always been disappointed with the performance of my limited-slip. Whenever I spin my tires in dirt or gravel only one tire spins. There have been times when taking off on wet roads my truck has just sat there and spun. I have to feather the throttle just to get the truck moving. I have to put the truck in 4WD just to get around my snow-covered driveway. The list goes on and on....

There have been several solutions given for this problem. A couple guys have had their stock limited-slips rebuilt with more clutch disks. Others have replaced the factory units with aftermarket limited-slips or lockers. The solution that interested me was replacing the differential oil with Amsoil synthetic oil. Many people report this change makes the factory limited-slip work like it should. This appealed to me because it's cheap, and I can do it myself.

I ordered 4 quarts of Amsoil 75W140 gear oil from Bob Riley, a common poster at It's hard to find it on the website, so you can find it by clicking here.

I removed the rear cover. It uses a 1/2" socket.

Scrape off the old gasket and clean the cover well. I used Permatex to reseal the cover. There is no conventional gasket made for this housing. I let the Permatex 'skin over' while I cleaned the old gasket off the differential housing.

Here are a couple shots of the Ford limited-slip differential. Clean the old gasket off this as well. My housing and cover had a few burrs, so I used a file to lightly file them smooth.

I used a hand pump to put the fluid in the differential. These can be found at most auto parts stores, and don't cost alot. They screw onto most gear oil bottles.

That evening I went to my brother's farm to throw some old porch railing into his sinkhole/burn pile. The trail through his pasture contains a couple good tests for the differential. The flat part of his field holds water, so it's often marshy. There's also a valley where water runs off through the bottom. There are also some semi-steep hills that can be tricky when the grass is wet. In the past I've often slid the truck into 4WD just to be on the safe side.

As soon as I passed through the marshy part of his field I could tell the limited-slip was working better. No wheelspin at all! Down through the valley and up the other side with the same surefootedness. After I dumped the porch railing I goosed the truck going back up the hill. The rear of the truck instantly kicked to the left...the sure sign of limited slip. I looked back to see two beautiful bare tracks in the field (don't tell my brother... he he).

I'm now a believer in the Amsoil.

Update (5/2002) - After having used the Amsoil for a few months I have to modify my earlier statements. The limited slip still works fine in 'light duty' applications...wet roads, gravel, etc. HOWEVER, whenever you need the limited slip the most it still doesn't work. I'm talking about "pedal to the floor" kind of action. I tried to pull out an ambulance that was crossed up in the snow...the limited slip didn't work. I pulled off the a narrow county road to let a school bus by and the tires sunk about 6" into soft dirt...the limited slip didn't work. I'm still glad I changed to the Amsoil. It does make the limited slip work better. But this is another instance where I have to give Ford the rasberries for poorly designing something on a 'Superduty' truck.

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