Many of the Pros on this site use Mobil 1 Synthetic (75-140). So I did too. From what I read you don't need a friction modifier as its already has some in it. MoyockPowerstroke
showed me a site (www.partsguyed.com) that sells an Ford aluminum axle cover for $50.00. I ordered that too as I do a lot of towing and I found the axle fluid gets very hot. The seller is on this site somewhere.