Bit of a thread revival, I ended up going for a 50:50 mix of Torco MTF and RTF. The diff is amazing on track but ever so snatchy/clunky in every other scenario so I've caved in and put a little Torco type F friction modifier in, literally put 20ml in and it's made the diff silent! Just been for a test drive and I'm well happy with it so I'll see how it goes from now on.