Since all YS engines have a NIKASIL like sleeve, the Castor oil protection so revered in ABC/AAC/ABN engines is not necessary.
Also, it (the Castor oil) is said to gum up the fuel pump diaphragms and valves.

Use 20/20 YS, or 30% nitro heli fuel for best performance, in YS engines.
Use the full 20% synthetic oil, in all YS engines.

For any other four-stroke, 10%, sometimes even 5% nitro is good, with a synthetic/Castor oil mix.
For an improved idle, response and power, up to 20-25% nitro can be used.