What happens to oil when the engine is shut down?

When the engine is shut down, the oil settles in the sump. This is important for ensuring accurate measurement during pre-flight checks or maintenance. The design of the engine allows for oil to flow back to the sump when not in operation, making it easier to gauge the oil level. This reliable measurement is crucial because it indicates whether there is enough oil available to ensure proper lubrication during the next engine start-up.

The other options do not accurately reflect the behavior of oil in a shut-down engine. Oil does not evaporate completely or drain back to the oil cooler, as those processes are not part of the engine's operation when it is not running. Additionally, while some oil may be returned to the tank in certain conditions during operation, when the engine is off, the oil is primarily resting in the sump area. This settling allows for more consistent and reliable assessments of oil levels for the operator.