Have a 2001 4X4 screw going to be changing the rear end oil, question is do I need friction modifier also?

 

No need if not limited slip. Friction modifier increase friction to prevent clutch from sliping. The best friction reducer is Molybdenum Disulfide Coatings (MoS2) that bonds to metal use this only on open differential. It is used to break in a flat tappet camshaft.

 

^^ Not sure why you mentioned MoS2, then say don't use it? Seems a bit confusing to the OP.

Grim... look at your door tag. If the axle code has a letter in it, you indeed will want to use friction modifier because your truck has limited slip. If it has two numbers, then it is not necessary. See the pic... I circled where the code is.

 

What I mean was not to use MoS2 in a Limited Slip tranmission that requires friction modifier.

 

For once I understood w0lvez.
If you have LS, use a Friction modifier (The Ford XL-3 stuff works best IMO), even if the gear oil says it has it, it is not enough.
MoS2 is primarily used in engine building as said, and is not a friction modifier but a friction reducer helping with reducing gear wear. Never heard of it being used in the axle before, but may be a good idea in an open diff to help reduce wear.

 

It reads better now. He went back and changed it.

I agree though, Ford XL-3 is the only way to fly.

 

EPA has done a lot of changes in oil formulation. Using todays assembly lube and engine oil to break in flat tappet is causing camshaft failure upon start up. I'm still using the same viscosity used by the factory. 75w140 is too squeeky and causing too much drag. Waste of fuel

 

I've been using Valvoline 80W-90 in mine, front and back. It's been great stuff.

 

I'm not using multi viscosity gear oil. SAE 90

 

Thanks mine is the same as that one I put in the modifier and it definitely helped thanks