CAUTION: Model engine fuel is extremely flammable and burns with an almost invisible flame. It should be store in a cool, dry place away from sources of open flame or heat! Methyl alcohol (AKA wood alcohol) is a poison! It should never be taken internally and should be stored in a child proof location.
Methyl alcohol and nitromethane have different burning properties. Generally, the more nitromethane, the more horsepower. There are some limitations to this. Most model engines do not have ignition systems. The timing of the engine is a combination of compression ration, the flash-point of the fuel, and the glow plug characteristics. Change the fuel mixture and the timing will change. If it changes enough, horsepower will decrease and the engine becomes difficult to control. It may even be damaged. Sport engines are designed to operate best with about 5% to 15% nitromethane, 15% to 20% lubricant and the rest is methyl alcohol. Engines can also be designed for the extreme cases. Some FAI control line engines are optimized for NO nitromethane at all. Some racing engines can operate with as much as 50% nitromethane content.
Almost all modelers will start with a sport engine. This is a general purpose engine which uses a good quality commercial fuel containing 5% to 15% nitromethane and 15% to 20% lubricant. Most engines require a break-in period. 5% nitromethane and 20% lubricant is suitable for break-in and ordinary use. 15% nitromethane and 15% to 20% lubricant is used when more power is required. Note that even a small increase (3 - 5%) in nitromethane will improve flexibility, making the needle-valve adjustment less critical and improving throttle response.
Engine construction should be a consideration in your choice of lubricant and the lubricant content. Three types of engine construction predominate.
The ringed engine is similar to your car engine. The piston/cylinder seal is made with a thin ring of metal. This ring must be lubricated at all times. The lubricant must be supplied in the fuel. The greater the lubricant content of the fuel, the more lubrication of the ring. Ringed engines usually are easy to start and operate. They may not have the high speed performance of the other two designs. We recommend at least 20% lubricant or at least 15% castor oil lubricant with these engines.
The ABC and ABN designs have no ring. The cylinder sleeve shape is designed so that the walls of the cylinder are perfectly parallel at the operating temperature of the engine. When the engine is cool, the top of the sleeve is much tighter than the bottom. This is because the temperature of the top of the sleeve, near the cylinder head is hotter than the bottom when operating. This causes the engine to be sightly more difficult to start but gives it excellent high speed performance. The important thing here is the choice of sleeve plating. The chromium sleeve is much tougher than the nickel. It can withstand a higher temperature. The ABN engine will require more lubricant. In fact we recommend the use of all castor oil lubricant for these engines.
Despite all the information available on fuels and lubricants, you choices may be limited to what is available in your area. Model engine fuel is considered a hazardous material by the Department of Transportation (DOT). Transportation, storage, packaging, and labeling is heavily regulated. The cost of shipping a gallon of fuel throgh UPS can be more than the retail value of the fuel! This means your least expensive source of fuel is your local hobby shop. You are limited to the fuels that they carry. Don't fault the hobby shop here. They have as many problems getting fuel shipped to them as you do.
If the choice of fuels in your area force you to use a fuel which contains any synthetic oil, we suggest running your engine at a richer setting, which will allow more lubricant into the engine.