I replaced my 4wd actuator yesterday and noticed that the fluid in the front diff was a little low and mustard colored...perhaps old and perhaps with moisture??

the manual says 80w-90 GL5 but there are so many brands and types on the market with mixed reviews of causing problems with limited slip, etc but may not be an issue in my burb. 1998 K1500 suburban 5.7

this is my beach cruiser so it does all of its 4wd work in sand, year round. What brand / type of gear oil should I use in this situation?

THX