[mitchy]

Motor oil in the rear hub.I don't recommend that at all.
The Hub is not designed to contain oil.
Do you want oil oozing out of your hub onto the rear tyre.
The power increase would be minimal.
Keep grease in the hub for your own safety.

 

[mitchy]

How are you going to seal it properly?
When the oil gets hot and pressure builds the oil will force it's way through the "grease seals" in the bearings
on the wheel side.
The seals in MV hubs are grease seals not oil seals.
Oil seals are a different design with spring coils to contain oil.
How long do you think a needle roller bearing will last with a smear of oil on it at 200 plus kmph for 10 laps.
Do what you want.