Well considering you are RACING the car because you have a spool... The lightest weight possible so the car can go the fastest it can and not have to use horsepower to cut through the thick oil.
If you are not racing as suggested... any 80-90 and synthetic is always good here.
If it were a Posi (Limited SLip Diff unit) cones or clutch type then use mineral based gear oil NOT synthetic.
Hope this helps.
A spool does not require any special fluid ...