What properties gradually diminish in old oil as it thickens?

The cooling and lubricating abilities of oil are critical for maintaining engine performance and longevity. As oil ages and thickens, its ability to effectively lower temperatures and reduce friction between engine parts declines. This is primarily due to the breakdown of additives and the formation of sludge, which can inhibit proper lubrication.

Thicker oil can lead to increased friction and heat buildup, as it cannot flow as freely to the moving parts of the engine. Additionally, this reduced ability to lubricate can result in increased wear and tear on engine components, potentially leading to failure. Therefore, understanding the significance of these properties and how they change as oil ages is vital for maintaining engine health.

While other options may reference relevant properties of oil, they do not directly address the critical functions of cooling and lubrication that are most impacted by the aging process of oil.