What is indicated by the viscosity index (VI) of an oil?

The viscosity index (VI) of an oil measures how much the viscosity of the oil changes with temperature. A high VI indicates that the oil maintains a more consistent viscosity across a range of temperatures, meaning it will not become too thin at high temperatures or too thick at low temperatures. This characteristic is important because it affects the oil’s performance in various operational conditions; oils with a high VI are typically preferred for applications where temperature variations are significant. In contrast, oils with a low VI experience more dramatic changes in viscosity with temperature, which can lead to inadequate lubrication under extreme conditions.

In this context, the other options do not accurately describe what the viscosity index represents. The temperature range of operation, while relevant to oil usage, is not directly measured by the VI. The lubricant's ability to prevent wear and its load-carrying capacity are separate factors related to the oil's formulation and additives, not to the viscosity index itself.