It does the harm slowly if your careful, suddenly if you're not. You need that oil to help carry away the heat from the internals of that small powerful engine. 10% won't do it. Anything less than 18% is a hazard. The operative word here is "small". The large engines have enough metal to get rid of the heat, the little guys don't.

I know you guys over there, on the island and on the mainland tend to run less oil than we do but you also usually run less nitro.

Again these little guys use oil as a coolant, not just a lub. When you cut down on oil your cutting the coolant.

Also YS doesn't rely on blow-by for lower end lubrication like the other 4 strokes. The lower end is the first stage of the combustible charge accumulator. If the ring gets worn the resulting blow-by is going to kill your power even if the engine feels like it has good compression.