if you can get 5%, buy it, if not get 10%. you 15% is waste for a two stroke (for the average sports flying joe).
get 18% oil with some castor (look for wildcat XX% nitro extra, meaning 18% oil), do not get all synthetic. those tower engine are tight when you first start them (need a starter) also keep the engine above half throttlefor the first tank or the engine will quit in flight. have the instructor fly around for a few flights (at least two full tanks) before you get to training. the engine is great it just needs some time to break in.