MTL, pisswater thin, doesn't hold up very well.

What fluid does the diff maker suggest?

5w30 Synchromesh + 1/3 tube of Posi additive is what I used with my Kaaz, JBH also used it in Pebble. IMHO it would bee too thin due to the heat you will be seeing.

I've wanted to try 15w40 for diesels with their extra additives before it is phased out as well.

tGA, these are simple creatures. For what you'd pay for Torco RTF ($21/quart) you could use 3x Synchromesh ($7/quart) or 8x 10w30 ($2.50/quart).