I’m using a gear diff in my T4.1 RTR because that’s what it came with. I’ve tried 5000k xray diff oil which was too thick! So I went to 2000k which was still too thick. The track I run at is indoor clay TRCR, which has very technical turns, sweeper and an off camber turn. I ended up using regular black grease. My findings are that when it had diff oil it worked nice out of a turn but is not very good during turn-ins even with just black grease. I will end up just putting a ball diff, for one thing it is easier to tune, I think it also releases better during turn-in and grips better during straights if that makes sense.
I just don’t know why if ball diffs are so great, why don’t 1/8 4wd use them?