Just about done rebuilding my rear differential wondering what brand gear oil and weight to use and how much???????

Rear is 4 quarts. Popular choices on this site are Amsoil and Mobil 1 75W90.

"Rebuilding" your diff? What happened to it?

Moreover - if you can rebuild a diff, how is it that you cannot make up your mind on the type and amount of fluid to use? To be honest, I find that odd. Anyone that I know that has the expertise to do a diff job, either:
1) has clear preferences as to the fluid choices, and knows where to find the info for refill capacity
or
2) realizes that as long as fluid spec's are met, it's moot, and fills to the correct level


Bottom line:
You may use ANY brand of GL-5 rated hypoid gear oil, either dino or synthetic, in a few different grades, per the OEM of the diff itself (Eaton).

The POPULAR choice is 75w-90 synthetic, from many brands.

I'll let the mythology and rhetoric rage on from here regarding brands and such ...

3.7 qts is the capacity per AAM (about 1/4 to 1/2 inch below the fill port) Best to break it in with dino GL5 hypoid gear lube rated for use in Limited Slip diffs. After the break in period is complete drain and refill with your lube of choice (that meets specs). I normally run 75w90 mobil synthetic.

I changed my Diff fluids and the rear i put 4 Qts of Mobil 1 75w90 and it was a tad bit below the fill plug.