I have a 2010 Tahoe 4X4 and the fluid recommened for the transfer case is Mercon VI , it is a single speed transfer case. I use to own a 2006 GMC ,Envoy ,4X4, two speed trans. case., and the transfer case fluid recommended for that truck was Auto-Trac II fluid . #12378508, oil is blue in color. It seems to me the Auto-Trac II would be better for the Tahoe . Here is what it says on the plastic Qt. bottle.---- For four wheel drive vehicles with automatic transfer case. Withstands extended high temperature operation. Improved oxidation resistance. Does any one know if this is a better quality oil than the Mercon. I think I read somewhere it is better oil to resist corrosion of aluminum cases.