Engine lubrication basics When the engine is shut off and not running, where is the bulk of the engine oil stored/collected?

Introduction / Context:
Knowing the oil circuit path is essential for correct servicing and diagnosis. Most passenger engines use a wet-sump system where oil drains by gravity to a reservoir integrated into the crankcase.

Given Data / Assumptions:

• Typical wet-sump engine (not a dry-sump race engine).
• Pump is a positive-displacement unit drawing oil from a pickup in the pan.
• After shutdown, oil drains back from galleries and head to the sump.

Concept / Approach:
The oil pan (sump) is the lowest part of the crankcase and serves as the oil reservoir. When the engine stops, residual oil in the head and galleries drains down through return passages into the pan, from which it is recirculated on the next start.

Step-by-Step Solution:
Identify system type: wet sump.Trace path: pan → pickup → pump → filter → galleries → components → drain-back → pan.Therefore, the stored location at rest is the oil pan.

Verification / Alternative check:
Service procedures for oil changes involve draining from the sump plug; this confirms the reservoir location.

Why Other Options Are Wrong:
The head retains some oil but not as a reservoir.

The pump does not store oil; it merely circulates it.

Separate tanks are for dry-sump systems; not typical for road cars.

The filter housing contains only a small volume.

Common Pitfalls:
Overfilling the pan can cause aeration and high consumption; a dented pan may restrict the pickup, causing low oil pressure.

Final Answer:
oil pan