First of all, 15% nitro fuel is not suitable for any YS engine. 20% nitro, 20% synthetic oil and the balance methanol is the recommended mix. The heli blends with low viscosity, high percentage oil packages also work extremely well. Cool Power 30% heli fuel is an example.

Whenever you add oil to an existing nitro percentage fuel, you also cut the nitro content. Better to add oil and nitro if you have to do it that way.