Do you have a local hobby shop LHS? You can use 10%, save money, and it will run just fine. I buy Omega 10% from Hobby Town, add 4 ounces of castor oil and few drops of Armour All. Problem with ordering fuel online is that they really hit you with the hazard shipping fee. Buying at a hobby shop it's not as bad because they buy in quantity and pass along the savings. I've never bought 15%. It does depend where you live though. If you're at a higher altitude or fly competitively, then maybe I would agree 15% is needed. But I've used 10% from the '60s with all my engines and its always been fine. I do like to add 4 ounces of castor to a gallon to bring up the oil content protection, and a few drops of ArmourAll for de-foaming. Long story, but its a controline formula that has always worked for me. I have never fried an engine, the extra castor effectively protects/cools all engines. These 2 stroke engines cool by passing castor right through them. Yeah, I know, they call them "slimers", lots of oil to clean up after a day of flying. But I consider coming home smelling like castor = to having a great day.