IMO all four strokes perform better with 14 to 18% oil, but warrienties may be voided, thought I don't know how they would know. Four strokes do not need as much oil because the oil remains in the crankcase till it fills up and the excess is dumped out the breather. Some may have lube problems on the rockers with low oil content, thought mine never did. A small amount of castor improves lubrication and doesn't affect the valves. Saito used to require all synthetic oil, I think they now recommend a small amount of castor in the mix.

YS is a bit differant as all of the oil goes throough the crankcase and should have at least 18% oil, 20% is better. I think castor is bad for their diaphram. Sure some YS owners will give you the right info.