Several possibilities of gear oil leak from inside the final drive/rear wheel hub:
1. There are 6-8 "cover bolts" that hold the inside cover of the final drive on (these are inside the rear wheel hub and around the perimeter of the final drive under the rear brake shoes. I once had one of these come a bit loose and seep oil. simple cleanup and retighten fixed it.
2. The rear brake actuating rod that goes through the final drive to actuate the rear brake shoes has 1, 2 or 3 O ring seals on it. These can sometimes deteriorate/leak. I have also discovered that if the shaft has some "play" in it such that you can push it in/out with the rear wheel off and rear brake shoes removed, it might be moving such that the o-rings aren't staying in proper place to seal. Making sure that the CAM and washer on the inside are flush to the inside cover of the final drive, and putting the splined lever fully seated on the outside part of the rod prevents the side-to-side play and stopped a leak on my bike.
3. The large seal which goes around the drive splines that engage the splines on the wheel hub can sometimes fail/leak, but this isn't very common.
4. The "cap" on the very top of the final drive housing must be able to "breathe", or else under running conditions when the oil, etc. heats up it can become pressurized, and try to force oil out any weak points (Seals) in the system which otherwise wouldn't leak. It may be clogged.
5. It could have been overfilled, combined with #4, or not.
6. Either the drain plug (bottom of final drive) or "check" plug (horizontal/back edge of final drive) is loose, or bolt hole stripped, or crush washer needs replacing.
Most of the above items aren't very serious and easily fixed, but if you get oil on the brake shoes you'll likely contaminate them and have to replace them. Recommend you clean it all up, check careful, take for a little spin and then recheck for where it appears to be coming out of - we can help you from there.
