Posi Additive

Somebody here questioned why I recommended the Motorcraft (ford) additive, when Mopar also offered a friction modifier.
I answered that I haven't tried the Mopar stuff, I HAVE used the Motorcraft, as have all my friends, one of whom owned a rearend shop. We KNOW it works, that's why.

The fact that I think it's cheaper than the Mopar stuff helps too.