What does the term 'viscosity' refer to in engine oil?

Viscosity refers to the thickness and flow characteristics of a fluid, including engine oil. It is a crucial property that determines how well the oil performs under various temperatures and conditions. Higher viscosity oils are thicker and flow more slowly, while lower viscosity oils are thinner and flow more freely.

In the context of engine performance, viscosity directly impacts lubrication, wear protection, and the ability of the oil to reach engine components effectively. As engines operate and heat up, the viscosity of the oil can change, which is why selecting the appropriate viscosity rating for specific operating conditions is important to ensure optimal engine function and longevity.

The other options describe different properties that oil may have, but they do not accurately define what viscosity is. For instance, the ability to expand under heat relates to thermal expansion, resistance to combustion concerns the oil's flammability, and cooling capacity refers to the oil's ability to dissipate heat, none of which are direct definitions of viscosity.