Sometimes a good old fashion oil change with dino oil works fine to fix slipping or sticky clutch problems. Any time I feel something weird with the shifting I change the oil and it usually goes away. These bikes need the oil changed pretty frequently anyway. I've never added anything to the oil after the change, maybe add some cleaner just before. I do mostly city riding with lots of shifting, no problemo. If it's anything more than that, oil won't fix your problem you need to start in on the clutch, cable adjust, new cable, new clutch, etc.