I also run now the 3R diff for the first time.
Kit oil is really not for a RWD. Probably they just changed the label for the M06...
I changed simply to Tamiya soft shock oil without cleaning it to good from the kit oil, but its much lighter now and feels already quite good to me. Maybe slightly more tight than my Ta03 ball diff I run outdoors.
I will not change back to kit diff, I guess, but try even thinner oil and not greasing the outdrives (which I did when assembling).
My plan is to have it just before spinning freely when turning one wheel.
But the track will show the best solution, I guess...
What I really can recommend are the new Tamiya carbon shock towers, which make especially the rear tower less flex, and they are not too expensive. Also I have now 2 more holes in front for the dampers, definetly next tryout to see effect on turn in!