Mobil 1 LS 75W140 with friction modifier
 

Tried a search and no luck. Just bought a 2006 F-150 4X4 with 98K. Going to have both diffs serviced. I already ordered Mobil 1 LS 75W140 for the fills. What kind of luck have y'awl had with running this gear oil in a limited slip rear without having to add more friction modifier?

It's easy to find out.

Change the fluid out. If you get clutch chatter, it's time to redo it with some 75W140 with some XL-3 from Ford.

Check the owners manual. I think Ford recommends 80W-90 for the front diff.

I think they do. But Motorcraft 80W90 is non-synthetic, and I have read a few complaints about Mobil 1 75W90 being too thin. I have never read about any problems running synthetic 75W140 in the front, and it looks like there may be a few advantages.