I will be changing out my rear diff. fluid soon with amsoil 75w-140. I have Limited slip so I will need to add friction modifier. What Friction modifier will allow my LS to perform the best?

Should I use the amsoil friction modifier since I am using amsoil gear oil, or use motorcraft FM since its a Ford truck.
Let me know what you guys think. - Thanks

 

It doesn't matter...

I recommend:

Additive Friction Modifier - Ford Part # - XL-3.

 

Are you sure it doesn't already have a friction modifier in it? I know RP has it in their gear oil already..

 

Royal Purple and Lucas are the only gear oils I found in which it comes pre-added.

 

From what I hear is usually best to add it anyways.

I just want to make sure my limited slip still works to its full potential and locks up the correct wheel as much as possible, because I live in MI with bad winters and I off road in the sand dunes often.

 

amsoil 7590 sgd here with their additive-locks up perfectly in the winter as designed-phil
btw-rp says it will work in locking differentials but when i asked them if their oil met the specs of ford's additive they said no but they didn't have any issues in fords-phil

 

I used straight Royal Purple 75w-140 in my truck when I got my 4.88s done and I had no problems with my LSD.