My 07.5 CC short duramax owners manual says that the rear axle calls for synthetic 75W90 that meets GM 9986115 spec. Havent found a bottle that has that spec on it yet. I'm used to seeing the GL5 requirement but not that spec. What are you guys using and what about the warranty? Funny, my 07 Chevy Uplander manual doesnt even give you the normal lettering requirements like SG/SL/SM etc. It just gives you GM 91** spec as required oil in 5W30. But I'm still wondering about this rear fluid. I've always used Mobil 1 in the rear of my Tahoe and even used some synthetic 75W140 Super Tech (Walmart) in it as well. Never a problem. Does mobil 1 meet the spec for the rear in the Max? Thanks in advance.