I'm running 2 YS45 2c. with Byron 5%nitro and 20%oil with excellent results. Very low idle and perfect transition ev'ry time.

A buddy of mine is running his YS91 with the same fuel as his other engines (Power Master 10%nitro) and he gets the same results.

So if I were you, I would try the fuel you're using normally cause if you're like me, I hate to have to carry many different kind of fuel and if the engine doesn't run as supposed to (perfectly), then and only then I would switch to 20\20.

But saying a YS can't run good on lower than 15% nitro doesn't convince me. In fact, I would pay more attention to the oil content than to the nitro.

Just my two cents opinion.