Use a quality straight (minimum) 90wt (I like their 140wt myself) and only use the OE Ford friction modifier... not the junk off the shelf at the local parts store. There is a big difference in the composition of the fric mods available and Ford makes the best parrafin based stuff there is... and RedLine makes the best synthetic based stuff there is.
I might also recommend that you use a 'better' differential lube as well. The cheaper parts store brands (to include Valvoline) do not tend to homoginize with the friction modifiers very well. When they seperate... you get noise.
My personal recommendation for lube is in this order:
1. Schaffer, #209 with the suspended moly
2. Torco, RGO 85/140
3. LE... Amsoil... and all the rest.
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