I have several YS 4C and I use either of 10% Coolpower or YS 20/20. I have unlimited idle control on both fuels.
A carefully broken in engine will idle much better on most reasonable fuels.

When moving from 10% to 20% I have to richen the needle.

20/20 is great as it makes my non-smoke planes seem like smoke system planes; I always try and run my engines on the rich side but have never compromised on power.

Great Engines these YS.