viscosity index

The viscosity index is an arbitrary number used to measure how much a lubricant's viscosity changes with temperature. A higher viscosity index indicates that the lubricant's viscosity changes less with temperature fluctuations, which is desirable for consistent performance.1

Viscosity itself is a fluid's resistance to flow or change in shape. Think of molasses versus water; molasses has higher viscosity. This internal friction within a fluid is crucial for lubrication, affecting how easily fluids move through pipelines or are used in processes like spraying and coating. For many fluids, viscosity is a constant value at a fixed temperature, known as the dynamic or absolute viscosity.2

Liquids generally become less viscous as temperature increases, while gases become more viscous. This property is important in selecting lubricants, as their performance can vary significantly with operating temperature.2