80w90 is what it specified for both the differential and transfer case. Will 75w90 work OK in either one of the two without causing any problems? Also I was wondering if I need the limited slip additive for the differential or transfer case. Doing preventative maintenance and all help is appreciated. This is my mothers. I own a Volkswagen and tend to buy German brands for my car. I might get her some Liqui Moly 75w90 if I can use it instead of 80w90. If it is like motor oil you can usually get away with a slightly different oil in this way but don't stray too far from the specs.