Spool up front will give you less turn-in steering, but you can get on power earlier and it will pull you through the corner nicely without the rear stepping out. So you'll have more on-power steering and a very stable car. On slippery track (low traction, dusty, wet) don't use spool, the car will not want to turn and will be slower overall. Medium or high traction, go for the spool instead of the diff.
Rear diff oil - generally, the less traction the track has, the lighter oil you should use. More traction, heavier oil. Heavier diff gives less turn-in, but more on-throttle steering and of course more forward traction. If your diff is too heavy for the traction, the rear will want to step out when powering out of corners. If it's too light, the car will be unstable under braking in and understeer while powering out.