I tried 15% nitro and didn't see enough perofrmance difference to justify the higher cost I only use 10% now. As far as synthetic vs. castor oil the synthetic is more prone to absorb moisture and could cause you more problems if you live in an area that has high humidity and you don't use after run oil. I use a mixture of 50/50 synthetic /castor with good results. Which ever one you use, best to make sure that its at least 18% oil content of the fuel. I am surprized that they sell fuels that have less than this but they do so check it before you buy it. [8D]