Rear differential Oil ?

I second what @ThunderOne said. Replaced both of my diffs with ECGS pre assembled units and have spend a fair amount of time with them over the phone. They recommend Lucas 85-140 non-synthetic oil for all applications. This is the product that they have found to work best through thousands of differential rebuilds.

An additive, to the best of my knowledge, is only necessary when a LSD is being utilized due to the clutch plates inside. If you are e-locked, an additive is not required because it's a mechanical/electrical system. Personally, I don't understand why you would try to cut a few dollars in any lubricant when you're dealing with parts that cost much more to replace but that's a personal decision.

 

I've always run the heaviest weight oils I can get away with. That's why I run the 75-110 Amsoil in my diffs and the 10-30 in the 2.7 and it "can" get down to -20 degrees here as well as stay at 0 degrees for a week at a time where it is normally around 20 to 30 degrees here most of the time in the winter. It does get down to -30 for short periods as well. Maybe a few days. I haven't had any problems. I did install a battery heater as well as an oil pan heater for this next winter.