I had this problem with the engine running and transmission oil leaking from the gear box through the shaft and out, it dribbled out faster dependent on the speed/revving . However in the end I had the help of Ray Tew who worked at Hitchcock's previously. My fault was there is a type of Archimedes screw for oil to on the shaft if I recall, there had been a batch made incorrectly Ray told me and this pumped oil out of the gear box; or it could possibly be worn, it was replaced and the problem was solved. Hitchcock's (Alan) has Ray's number if you need any more information. Just a thought!
There isn't much Ray don't know about Royal Enfield's.