My FSM does not mention an additive. The Redline diff oil I use is LS compatable and I have never needed any additive. I don't think it will hurt though, but not sure why you would need it unless there are operational signs. I will say that the addition of LS additive to the tranfer case makes a world of difference to its operation, especially in cold weather. The FSM does not say to use an additive there either. I have used GM additive in the transfer case and just recently used off the shelf Advance Auto product, can't remember the brand. I used a length of clear vinyl hose pushed on the end of the tube to get it in the transfer case. A little pre-drain to make room also made it easier.