FAA Powerplant Written Practice Test

What type of oil do most engine manufacturers recommend for a new reciprocating engine break-in?

1. Synthetic oil

2. Ashless-dispersant oil

3. Straight mineral oil

4. Multi-grade oil

The correct answer is: Straight mineral oil

Most engine manufacturers recommend straight mineral oil for the break-in period of new reciprocating engines because it facilitates the proper seating of piston rings and helps remove any machining residues. Straight mineral oil has a simple composition that allows for enhanced ring wear-in by providing good lubrication without the additives that are found in more advanced oils. During the break-in phase, the goal is to create a proper surface contact between the piston rings and the cylinder walls, which is most effectively achieved with a lower viscosity oil without dispersant additives. This ensures that the oil can easily flow into the small gaps and help the rings seat properly. After the break-in period, pilots typically switch to ashless-dispersant oil or other formulations suited for regular engine operation. Other options, while valid under different circumstances, do not serve this specific purpose. Synthetic oils, multi-grade oils, and ashless-dispersant oils contain additives that may inhibit the ring seating process during the critical initial hours of operation. Therefore, the choice of straight mineral oil aligns best with the manufacturer's recommendations for optimizing engine performance during break-in.