I agree with Dog, no additives in our bikes because of the wet clutch. You're just asking for trouble. Once a clutch starts slipping you might as well replace the clutch plates. Just use synthetic oil and a good oil filter. A cap full of Marvel Mystery oil in your gas each tank full to keep your fuel side lubed and you're good for a very long time. Think of a 2 stroke with no crank case. You add 2 stroke oil to the gas to keep your cylinders and rings lubed up. Well on a 4 -stroke the oil only gets to crankside of the piston and the top end valve stems. If you add a little Marvel to the gas it will help keep the combustion side lubed on the compression stroke. At least that's what an old mechanic told me a long time ago. I've been using Marvel in all my engines big and small for 30 years without any problems.
