+1 on the lighter diff oils. not just for lower traction but for smaller and tighter tracks too. if you have 180 turns around a singe tube then 5/3 can really get around the turn tightly and let you power out.
high bite clay I'm liking 7/5. black dirt with dust on top 5/3.
5-6' turn radius, 7/5, 2-3' turn radius 5/3.
obviously, you want to tune for the majority of the track.