What maintenance task is essential for prolonging the life of an engine?

Regular oil changes are crucial for prolonging the life of an engine because engine oil acts as a lubricant for moving parts, reducing friction and wear. Over time, oil can become contaminated with dirt, debris, and combustion byproducts, which diminishes its effectiveness. Changing the oil helps ensure that the engine operates smoothly and efficiently, thereby reducing the risk of overheating and mechanical failure.

The other options, while they may contribute to overall vehicle maintenance, do not have the same critical impact on engine longevity. Regular cleaning helps with appearance and can prevent rust, but it doesn't affect internal engine function significantly. Replacing spark plugs every month is excessive for most vehicles and not typically necessary for optimal engine performance. Periodic engine shutdowns might be relevant for operational efficiency but aren’t a maintenance task that inherently aids in prolonging engine life. Thus, regular oil changes stand out as the most vital maintenance task for engine health.