Varys by brand for me. What engine is it? If OS, I fly them out of the box. They are awsome. Only the 91FX needed breaking in. Webra takes all kinds of break in. Usually I break an engine in with 10 % nitro, 20 % oil fuel and run a tank or two pretty rich, then lean it, but not all the way and fly it. Has been working well for a long time. Never had an endine go bad.