i wouldnt trust tower hobbies, they may have messed up, nut the official ofna site i would trust. basically, 10-15% nitros are not very common, my lhs doesnt even carry it much. it usually doesnt matter what % of gas u use, i take whatever is available for my .18 associated mgt and my .28 ofna ultra. 10-15% are going to not run as high rpm's, 20% is the average, and 30-33% run very high rpm's. i would take the 20% if i were you, it has a perfect balance and not overkill especially if its a new engine.