It says on the bottle that it takes care or open and limited slips. Do I still need to put a bottle of the additive in?

i do and believe it's better that way. i use the gm additive, nothing else seems to of ever worked as well for me as the factory stuff does.

I use synthetic Castrol axle oil in my BW posi, that says no additive necesery.
But I still use one bottle GM additive as well!
I have the feeling it runs somewhat quieter!

This is accually for my GF's RS with then 10 bolt clutch type posi. I know that on mine I used to use the additive on my 9 bolt cone type posy but it seemed to slip. I cleaned if all out and put in straight weight oil with no additive and it was fine since it seemed the cones accually needed friction to grab. I know thats not the best thing and I should learn how to shim it but rearends scare me lol.

I use Mobil 1 with GM additive.

JamesC