Ok, Eaton says to not use synthetic in their TrueTracs. I usually run 75-140 syn in my Diffs.
Since Dino oils only come in either 80-90 or 85-140, can I go 50/50 with these to get a little heavier mix?
Or should I just go with the 80-90?

4:56 and 35's

 

I wouldn't trust an answer to that question from some forum poster unless he happened to be a petroleum engineer. I don't think mixing oils would work like you think. For my Jeep, I'd use the 80-90 in the front and rear. If I towed, I'd use the 85-140 in the rear only. That would be as close as possible to what my '07 owner's manual specifies.

 

Eaton states to use a gl-4 or better conventional gear oil. They don't give you a weight recommendation. 80w90 shouldn't cause an issue. GL-5 80w90 should be easy to find.