First off i know there are probably 100k plus different answers to what I am about to ask but I will ask anyways. I am changing my rear diff cover and oil this weekend on my 04 FX4 3.73LS, and my manual says use 75W-90 gear oil, ok great! EVERYTHING i read on these forums says everyone is using 75W-140 with no mention at all about 75W-90. I have always heard go by the manual on this sort of stuff. I am going to use Mobil1 Full Synthetic LS and add a tube of the Trans-X mod. no matter what. So i was just wondering if anyone has any input on this

 

The manual for my '05 says to use 75W-140 for the rear diff. You may have an older copy/revision of the manual; you can download the owner's manual from Ford.

 

Isn't 75/90 what you use for the FRONT diff on a 4x4? In my rear diff I put 75/140 (2x4)...used Amsoil severe gear. Remember to soak out the sump in the bottom of the differential. Just saying.

 

My manual says 75W-140, and that's what the mechanic put in Tuesday when he replaced the rear pinion seal. Mine has the 3.55LS fwiw, but it'd probably be worth it to look at the newest version of the manual if Ford's got it.

 

Yep, the manual for my truck says 75W-90 regardless I went ahead with RP 75W-140

 

I just changed the fluid in the rear diff of my 04 Lariat and used Royal Purple 75W-140 with two bottles of Motorcraft friction modifier. Solved the rear end shudder I was experiencing almost instantly.

 

I didnt add any FM with the RP, but I have not had any chatter, shudder, etc since Saturday. I may add some later if i hear anything-Thanks

 

When I just put on my Mag-Hytec cover on last week I used the mobil 1 70 140 and 2 bottles of the modifier since it held 4.5quarts of oil. No problems.

 

That is what I did, i just got my Mag cover on also. Did you use the Trans X FM or Motorcraft?

 

I only used a single bottle of the friction modifier in my diff, and it calmed it down a great deal - but I still get chatter occasionally - I think I am going to slip under there, pop the plug and squirt as much of the second bottle in as I can and replace the plug real fast...