If oil has a low viscosity index (VI), what does it indicate about the oil's viscosity?

A low viscosity index (VI) indicates that the oil's viscosity changes greatly with temperature fluctuations. The viscosity index is a measure of how much the viscosity of a fluid changes with temperature. High VI oils maintain a relatively consistent viscosity across a range of temperatures, while low VI oils experience significant changes.

For oils with a low VI, as the temperature increases, their viscosity tends to decrease more dramatically. This means that heating the oil can result in it becoming considerably thinner, which can affect its performance in applications where consistent lubrication is critical. Conversely, when the temperature drops, the viscosity increases considerably, which could lead to issues with fluidity in colder conditions.

Understanding the implications of viscosity and the viscosity index is essential for selecting the proper oil for various applications, as it directly impacts the efficiency and longevity of machinery and engines.