changing the diff fluid on my 79, but im not sure what grade diff fluid to use. also do i have to specifically use the gm posi additive, or would any brand posi additive (say from pep boys) do?

searched around the c3vr forums, but have not really found the specifics im looking for.

the haynes manual says to use a 80 to 90 weight oil gl-5 spec. most of what i find on the shelves are 75-90w, 85-140 and so on. i bought lucas 85-140.

would i make a mistake to use that?

Save yourself a lot of grief and buy the GM gear lube and the GM additive. The additive should go back in first, then add the lube up too the bottom of the filler hole. It is much easier to use an electric pump to remove the old stuff assuming you are not working an a fresh rebuild. If you are removing the old stuff, drive the car around the block first. It comes out much faster when it is warmed up. The GM additive seems to work the best at preventing posi-clutch pack chatter.
Once the new stuff is in, go to a parking lot and drive in circles with the wheel to the stop. Do it in one direction then go full stop in the other direction. This procedure will ensure that the disks in the clutch pack get the additive in the friction material.

Good luck,
Scott

85/140 GL is too thick(unless you've got a truck diff in there!). Go with the 75/90, and the posi additive. I have used posi additive from LubeGuard, and it works quite well. (The GM stuff is easier to get, for most people.)

thanks for the input guys- this is why im a c3vr member! 

im going with the 75/90w and the gm posi lube