Rick - I'm running gear diffs all around this year in my TC. I've tried running spools in different chassis and classes, they just don't fit my driving style or our track conditions.
That said, I'd stock fluids from 100,000wt to 2.5mil or putty. The idea is to get it close to a spool, but gain the extra turn in and stability, at the expense of a bit of exit speed.
As VooDoo said, if the car pushes too much off power, lighten the fluid. Like a ball diff, lighter fluid will allow the rear to rotate more easily. If you loop out when getting on power, lighten the fluid, because if it's too thick, once you get on power, both rear wheels are putting more power down, breaking loose. Lighter fluid would let it diff out more before breaking traction. Do the opposite if the car gets too loose off power, but pushes on power.