Just wondering the basics about it...what it is, what it does, where it goes (what parts use it and how much), where to get it (special ford fluid or if i can get it at the local parts store), etc.
4 or 6 oz. bottle of blue liquid that Ford uses and sells to go in the rearend diff. It is supposed to keep the gears quiet and aide the clutches for the TrakLok. GM sells something like it.
Despite common misconception, friction modifier reduces friction. Too much and you lose the ability to launch hard. Too little and you'll burn up the rear-end rather quickly.
If you're changing your diff oil yourself, talk to a shop you know and trust. Tell them your setup and how you drive. They should be able to tell you how much you want to add.