The small plane's oil hatch

The small hatch hidden on the top of the engine hood gives access to this pretty yellow cap used to probe the engine oil's quantity.

In a car, the oil level remains a boring needle living on the dashboard.

When preparing for a flight, we like to be convinced that our engine is in its best mood. It would allow, amongst other, to reduce the probability of an unwanted country landing. Thus it is mandatory to check that enough oil is put at the engine's moving pieces disposal so that it can be correctly lubricated and cooled down. Where fuel would be to the engine what food is to our body, oil would be its water, allowing it to work normally. And we'd rather be correctly hydrated.

Then, when the remaining quantity becomes too close to the preflight checklist's limit, it's better spending 5 minutes and getting dirty than hurting the engine's feelings!

Cap locked, hatch locked. Preflight checklist can go on.