Hello,
I just redid the rear and refilled with cheapo 8-90 gear oil like in the old trucks. Do I really need friction modifier? I didn't see any clutches or anything in the diff, simple open diff spider gears. do you get noises without the modifier? if so do you only get them coasting and slowing down in a straight line?
Thanks

my truck is a 2005 f150 xl with an 8.8 rear.
the noise I am hearing could be incorrect pinion preload, or lack of it.

 

I Guess I do not need it, I have a non slip rear code 19.

 

You don't need friction modifier, but you used the wrong oil - you need 75w140 full synthetic.

 

Even the most expensive gear lube is cheaper than a new rear end..

Yep, you need to change that fluid again...

Mitch