Cycle Magazine ran some tests on a 250 cc single, usung castor oil based lubricant at different fuel:oil ratios. As the amount of oil decreased, engine temperature increased, piston fouling increased, plug fouling decreased, and power output dropped from 29 HP to 26 HP (20:1 to 30:1 change un mixture, if I remember correctly), I suppose due to frictional losses. Their conclusion was to stick to the engine manufacturer's recommended ratio, not the oil manufacturer's. They didn't measure smoke output, though.
"Red Leader", Velo LE Mk II, Mk III. Honda Benly CD 200