In my experience the first place to look is the gearbox breather as mentioned by Ray - it's very common to leak from there.

I agree with MrR's advice - I have just located a mystery oil leak on my bike that had been bugging me for weeks.
It showed as a fine coat of oil mist on the rear brake pedal and around the RH clutch case, leading to drips off the crankcase centre joint.
I finally tracked it down with the talc method (after having taken off/resealed clutch case, oil cooler hoses etc) - there was a flaw in the clutch case just above the oil viewing window - I repaired it with araldite and ally foil and it's perfect now!

RX

You mention it being a race engine...ive seen these problems when they are up to temp, and under extreme load...cracks appearing in the crankcases...they only appear at high revs/heat/stresses.

Could also be a similar problem on the oil cooler...do you have any photos of the oily areas we could look at ?