I'd recommend adding
SeaFoam Engine Treatment to the crankcase, as per the instructions on the can. Run the bike, idling, on the center stand, for five minutes, shifting through all gears (let the wheel spin freely, but do NOT touch the throttle...), to warm it up to operating temperature (at 180 F, the oil will suspend crud so it is caught by the filter; no fan needed as it won't get above 200 F); then change the oil, and the filter. Your oil will likely be coal black, when you drain it (this shows how much varnish, and carbon, the
SeaFoam dissolved from within the engine, and the transmission...). After that, you will probably never need to treat the engine/transmission again, to clean it -- just change the oil routinely, and it will stay clean for 20,000+ miles. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII