Why is it necessary to let the engine reach operating temperature before draining oil?

Letting the engine reach operating temperature before draining the oil is essential primarily because it helps remove heavy particles and deposits that may have settled at the bottom of the oil pan over time. When the oil is heated, it becomes less viscous, allowing it to flow more freely. This flow helps suspend and carry away contaminants such as sludge and dirt that accumulate in the oil. By draining the oil at operating temperature, these unwanted particles can be effectively removed from the engine, contributing to better engine performance and longevity.

The other options might not capture the full rationale behind this practice. For instance, while ensuring complete oil drainage is important, it is the mobilization of these heavier particles that primarily leads to thorough cleaning during an oil change. Similarly, increasing the viscosity of oil is not beneficial for draining; in fact, the opposite is true. Lastly, cooling engine fluid is generally not associated with the oil draining process, as the objective is to remove oil while it is warm, not cool it down.