This what I ended up with for the 8.75 with Dana
Gear Oil, Synthetic Mobile 1 LS 75W-90 1 qt 22 oz.
Limited slip Clutch additive ATP AT-210 4oz.
Long story short. No reason to go heavier unless generating lots of heat.
A little less on the cold rating will help in colder weather.
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I don't think your in danger of breaking the axles, but the former head mechanic at Pheonix had a story about doing that at his first job. Apparently got to be real gentle getting things moving with an early Powerwagon's drivetrain in super cold weather and old time lubes.