I read soewhere that I do not have to but it was not a Ford or Amsoil source that said it. I dont want to ruin $35 in gear oil by adding something that might contaminate or change it.

S.O.

If you bought the motorcraft "ford" synthetic gear lube for your truck it already has it in it.
The ony way to make sure is ti get the serial # off the rear end and see what type of lube and additive is required theford parts department at the local dealer should be able to tell you what you need
Good luck
BIG

PS why are you changeing your fluid I have had 2 bad pinion seals so far but I tow alot of heavy stuff

As far as I am aware, friction modifiers are only needed with posi-traction or lockers.

Installing a new Limited slip.

BTW, just found on the Amsoil site that the FM is not needed. Will try it without it. If it chatters I'll add it.

S.O.