I bought one gallon of 20% nitro fuel to try in my two YS engines, there is so little difference between the way they run on 15% and 20% that I personally don't think it is worth it. I only use WildCat fuel now which has about 1.6% castor in it. There are some guys on here who are a lot more knowledgeable about nitro and its effects than I am. I don't run any engine without a little castor in the fuel. By the way I rode a 125 Maico for several years but was too old "30" when I started.