I use the Royal Purple synthetic 75W-90. It already has the friction modifier in it.
Further, and this is a big plus, the synthetic is WAY less stinky than the mineral based stuff and regular friction modifier. Regular stuff is gigantic raunchy dinosaur farts, synthetic is small, polite dinosaur farts.
Unless I'm trying to get a killer ET or something, I just fill the pumpkin until the fluid starts to run out of the fill hole. it is usually 2-3 quarts, depending on how much of the old fluid you got out.
If your diffy is new or freshly rebuilt it is advisable to find a large parking lot and do some slow figure 8s after install to spread the friction modifier through the clutches. I have found this procedure occasionally unneccessary with the synthetic as well.
Good luck.
Steve
