I just got done with doing my rear diff on my 02 screw and I used the valvoline 75-140 synthetic with friction additive. The more I have read online the more I'm wondering if I should have added more friction modifier.

What do you guy recommend?

Also, I know that it take 2.5 quarts. I added three and it still isn't up to the fill plug. Should I add more or quit?

 

I did the same as you. After several short trips I noticed some noise from the rear. I then added ford friction modifier and all is well now. I also filled to the fill plug.

 

The diff holds 5.5 pints unless you have the 10.25" rear (7 lug wheels). That one holds 7 pints. Fill to at least 1/4 inch below the fill plug.

No need to add more modifier unless it starts chattering.

 

When I replaced the axle seal and lube in my 2000, it took two bottles of friction modifier to quieten the clutches. That year the clutches were shimmed a little too tight. There was a TSB out on them that recommended replacing the clutch pack and removing 0.010" of shim to keep them from chattering. A second bottle of modifier worked for me. The man I sold that truck to recently sold it with 170k miles and the rear was still quiet and working great.

 

I have always filled the diff with one bottle of Motorcraft friction modifier from the top and then with syn gear oil till it weeps out the fill hole. Never had any noise issues.

 

Good advice.