I need to change the rear diff fluid on an 08 Silverado and 07 Avalanche, both with the G80, so it's not a limited slip. The GM spec for the fluid is 9986115. But, after looking on the back of all the different gear oils, none of them say they meet this specification. On the GM forums, a lot of people are using RP or M1 75w-90. All of them say they exceed GL-5 specs. Is it ok to run the M1 in my truck without any issues? From what I've read, it seems like it should be ok. What are your thoughts? Also, I thought I read somewhere that Mobil1 makes the GM gear oil. Is this true?