If you want to be safe with a YS engine then Yamada 20% from Model Technics has to be the way to go It's what I use in my YS63 and it's never missed a beat. I use Prosynth 10% in all my other engines and that's great also. I was chatting to Alan from Weston at Sandown and he recomended giving the Prosynth 20% a go with the YS engines. Not sure on this as the YS's are a bit fussy with fuel but it would certainly make it easier to buy from one place

My advice, use the Yamada 20%