I've been here a while (like a month), I read allot but do not post too much ...... I've got a 2000 GMC Sierra; I want to change the rear end fluid in it. I bought some Mobil 1 75-90 oil, but I'm not sure if I need to add a 'friction modifier' ... the truck has 4.10 gears with the locking differential. (I've got a 5.0 Mustang with the ford 8.8/clutch pack limited slip and I know it needs the additive, but I'm not sure about the truck)? Any help would be appreciated …. Also if you do not know, could you post me some good websites for GM trucks … I know a lot of great sites for the ‘Stang, but I do not know of any good truck sites (with good forums).