Cone style suregrip synthetic gear lube.

I also don't recommend synthetic. It's good stuff, but IMO, it's too slick to allow the cones to really "grab" like they should and instead, they just chatter as they are trying to grab. I like it simple. 80w90 and Ford friction modifier. Lastly, you need to get out in a large parking lot and do some gentle figure 8s to break the unit correctly, or to mix the gear oil in an already built unit.