FAA Powerplant Written Practice Test

What type of oil is typically recommended by manufacturers for breaking in a new reciprocating engine?

1. Synthetic oil

2. Mineral oil

3. Casting oil

4. Multi-grade oil

The correct answer is: Mineral oil

Manufacturers typically recommend mineral oil for breaking in a new reciprocating engine due to its unique properties that aid in the proper seating of piston rings and the initial wear patterns of engine components. Mineral oil is a natural lubricant derived from crude oil, providing sufficient viscosity and film strength to protect engine parts during the critical break-in period. During the initial stages of operation, mineral oil allows for better wear characteristics between the rings and cylinder walls. This is particularly important as the engine parts are still wearing in and reaching their final operating tolerances. The use of mineral oil helps ensure that proper debris and any contaminants generated during the break-in process are kept in suspension, minimizing the risk of scoring or undue wear on these vital engine components. Synthetic oils, while offering better performance under extreme conditions and enhanced stability at high temperatures, may not provide the same effective break-in characteristics as mineral oil. They can be too slippery and may not allow for the adequate wear necessary for proper ring seating, which is crucial during the break-in phase. Therefore, mineral oil is preferred to ensure that the engine components properly mate and function optimally in the early stages of their operation.