As said above; 4 strokes don't require as much oil in the mix as 2 strokes do so the "4 stroke" mixes typically have less oil. Then you have the oddball 20/20 that's 20% oil. When I run my 24 year old os 120 4 stroke (pre surpass) engine on the 20/20 it doesn't idle as smoothly or as low as it does with the regular 15% with 18% oil.

It's a very minute difference though so I don't worry about it, power with the 20% is quite good and it IS more oil so that's always good so far as longivity is concerned. I would only worry about higher amounts of pure castor and it sitting for longer periods of time giving the oil a chance to gum up. Regular running typically keeps engines clean in my experience unless you're in very hot climates like the desert. When I lived out there we ran things a bit richer and oil would tend to cake on the outside of the engine more than I see now in the more temperate climate I'm in.