I got one of the TA Differential Covers and when I put it on I'm going to need to refill it. Is there a different additive I can use other than the Motorcraft stuff? It's a little on the expensive side. I saw some stuff at Advanced Auto. I don't remember the name of it but it said Posi-trac or something like that on the tube. It was in the same section as the gear oils. Also, is it 75W-90 or 75W-140 that the Lightning diff takes?

 

O Rielly's sells their own.

 

I've used aftermarket modifiers with no trouble at all. I usually add a bit more ever since my bother took his rear end noise back to Dodge and the finally added like another 4 oz's and it solved the issue.

 

I picked up some from NAPA.

 

some people use the GM modifiers in the ford rears!!! and like 2 bottles to boot...

 

If I recall, our trucks have the Ford sythetic gear oil in them from the factory. It costs around $17 a quart . As far as friction modifiers go, you will only need it for clutch type diffs, if you change over to a detroit locker or tru-trac you will only need straight gear oil.