How often should the rear end diff gear lube be replaced? What weight is it and how much do I need? Also any name brands recommended? Thanks.

 

The owners manual basically covers it. 80w90 with no additional friction modifiers what so ever regardless of the rear end (if stock). Both the standard open and G80 gov-loc diffs use standard 80w90 gear lube. Synthetic 75w90 can be used as well.

There are a number of threads on this so search up for recommended oils.