Did a search but couldn't find a problem similar to mine.........so here's my story.
So over the last month or so the shifting had become very notchy...hard to get into and out of the next gear. Especially shifting down. When rolling to a stop the lever would not move. A blip to the throttle with lever pressure and it would finally shift down. That and a few other symptoms told me that the clutch was dragging (not disengaging)...baddly. I'm thinking something is warped pretty good inside the clutch. So I open it up and check the clutch pack. Fibers in spec, steels with a slight shine and everything flat. Everything looks good. But everything was dry....no oil on the plates. There was also a slight black sludge like substance on all the plates and on the basket. So I engage my Univac brain and try to think where it came from. Had to be from the fibers. But why. Dry plates+black sludge like substance=dragging/non disengaging clutch. Had to be the oil. So I clean and lube everything, throw it back in the bike and button up the case. The oil in the sight glass was SLIGHTLY below half way up. Well between the marks. So I topped it off to the top of the glass. Just a tiny bubble visible at the top. Started up and when for a shakedown ride. Shifted like butter. No dragging, no problem shifting down.
What I figure what was happening is there was enough oil for the pump to lube the top end but as the oil was being pumped out of the sump there wasn't enough left to keep the clutch wet. Even though the level is between the marks, it needs more. BTW, using 10-40 GTX
As always, YMMV.