On that friction modifier note... 99% of the gear lubes already have the required friction modifier so don't add more separately unless you know specifically the gear lube you are using does not have it. And that friction modifier is only (!) needed if your rear axle has a Trac-Lok limited slip differential.
It is hard to find a gear lube without that friction modifier since it is compatible with all axle types.
Your axles are not all that fussy about the exact viscosity gear lube so long as it is a GL-5 gear lube. Use a lower viscosity like 75W-90 or 80W-90 for most conditions. If you tow a trailer, use a heavier viscosity like 80W-120, 85W-140, etc.