What does the viscosity index of oil represent?

The viscosity index of oil signifies a specific characteristic of the oil related to its viscosity—the measure of its resistance to flow—at different temperatures. A higher viscosity index indicates that the oil maintains a more consistent viscosity across a range of temperatures, which is crucial for performance in various operating conditions. This characteristic is particularly valuable in applications where the oil will be subject to temperature fluctuations, ensuring proper lubrication and performance of the machinery.

In contrast, the thermal stability of the oil, its density, or indicators of oil purity pertain to different aspects of oil's physical or chemical properties and do not accurately reflect what the viscosity index specifically measures. Thus, recognizing the viscosity index as a characteristic of the oil emphasizes its importance in evaluating its performance and suitability for specific applications, making it the correct choice.