New aircraft small glow engines should be broken in (first runs) with about a 10% nitro fuel, containing at least 18% to 20% of lube. Some of the oil should be castor to help cool the motor. Depending on the type of Engine, there are two ways of breaking it in;

A non ringed ABC type motor should be kept warm, but not hot, by running it at a slightly rich 2 cycle. No peak rpm running for at least 1 1/2 tanks. Cycle the rpm up and down and let it cool of between tanks. When it will hold a steady and reliable idle and advance rpm smoothly it is usually considered broken in.

A ringed engine needs to be kept cool during the break in to permit the piston and ring to seat properly. Run this type engine very rich, in a blubbering 4 cycle mode, preventing it from getting hot. The head should be cool to the touch while it is running. Run it for about 5 to 7 minutes at a time, letting it cool for about 1/2 hour between runs. Many ringed engines require a gallon or more of fuel run through them before they are completely broken in.