FAA Powerplant Written Practice Test

The proper grade of oil for an engine is primarily determined by the operating speeds of bearings. This is because the viscosity of oil must be adequate to maintain a proper lubricating film under various operating conditions, including the speed at which the bearings rotate. Higher speeds generate more heat, which can thin the oil, requiring a different viscosity grade to ensure effective lubrication and prevent wear.

In contrast, other factors such as the type of fuel used, engine weight, and age of the engine may influence overall engine performance or maintenance considerations but do not directly dictate the required oil grade as strongly as the operating speeds of the bearings do. Therefore, focusing on the operating speeds ensures that the oil chosen will effectively protect the engine components under the specific conditions they will face.