I have an 04 f-150 4x4 with the 3.73 ls. My manual recommends 75w-90 but I read all over here to use 140. What difference will the 2 make?

 

What manual do you have?

Everything I have read says to use 75w-140 in the rear if you have the 9.75 or the 8.8 diff and the 75w-90 is for the front only. Make sure it is a GL-5 oil.

Since yours is an LS get a bottle of Friction Modifier from Ford and put in in first and then add the oil. If you have the 9.75 diff it should take one bottle of FM and 3 quarts of oil.

 

Ford specifies 75W140 full synthetic with friction modifier for all rear limited slip diffs.

 

This is my manual, should I still use 140?
And I already have the friction modifier, thanks to this forum.

 

I called the parts guy at Ford and he said to use the 140, so I guess that's what I'll do.

 

Owners manual does say 75w90, but Ford has since change their recommendations and now state 75w140 replace the 75w90.

 

I just came upon this thread and I'm glad I did! How often does the diff oil need to be replaced or refilled? My 07 5.4L F150 Screw with 4WD has 3.73 LS. She's allmost 24k and 2 years old, never touched the diff except accidentally bumpin into it with my shoulder installing a backup light.

 

Owners manual should give the time/miles. I believe it is 100k.

 

100k or when the LS clutches start to chatter.

 

Your manual says 75-90, do it! Mine says 75-140 and is a 2000 3.55 L/S 4X4. I have to put 75-90 in the front axle. Differences? I don't know but I have Amsoil severe duty 75-90 in my old car, 9" with a locker, 35 spline axles and 3.70 gears. Maybe it's for heavier duty stuff, I can't say. Either way, add a bottle (they are small) of friction modifier.

 

Awesome! Thanks guys!!