Viscosity Index of lubricants: a critical property

The viscosity index is a fundamental characteristic of lubricants, determining their suitability for a specific application. It quantifies how much a lubricant's viscosity changes with temperature variations.

Viscosity, often described as "thickness" or "stickiness," is a lubricant's resistance to flow. High-viscosity lubricants are thicker and less fluid, while low-viscosity lubricants are thinner and flow more easily. A lubricant's viscosity is influenced by temperature, pressure, and its intrinsic properties. 

Read our article about the viscosity of lubricants. 

The role of Viscosity Index (VI)

The VI indicates a lubricant's resistance to viscosity changes due to temperature fluctuations. A higher VI signifies a more stable viscosity across varying temperatures. The VI is standardized according to ISO 2909 and ASTM D2270. It is determined by comparing a lubricant's kinematic viscosity at 40°C and 100°C to reference oils.

By knowing a lubricant's VI and viscosity at one temperature, its viscosity at another temperature can be calculated using interpolation. This allows you to determine whether a particular lubricant is suitable for a specific application.

Relationship between VI and temperature

The relationship between VI and temperature is illustrated in the graph below, using lubricating oil as an example. 

The blue line represents a lubricating oil with a low VI (e.g., 80): it exhibits a significant viscosity decrease as temperature rises.
A lubricant with a high VI (e.g., 140) maintains a relatively stable viscosity over a wider temperature range, as represented by the green line.

Factors influencing the VI

The VI of a lubricant is primarily determined by the raw materials used to make the lubricant. Conventional mineral oils typically have VIs between 95 and 100. Highly refined mineral oils can achieve VIs around 120, while synthetic oils can reach VIs as high as 250.

VI in practical applications

Generally speaking, a higher VI is advantageous when operating conditions involve temperature fluctuations, as the viscosity of the lubricant selected will remain stable over a wide enough temperature range.

However, the necessity for a high VI depends on the specific application. For applications with minimal load and temperature variations, a low VI lubricant may suffice. Applications exposed to highly variable conditions – such as machines working outdoors or machines with highly variable process conditions – specifically require lubricants with a high VI.

Consulting experts

When selecting a lubricant for a new application or when you encounter problems with an existing one, consulting with a lubrication specialist is recommended. They can assess the application's specific requirements and recommend the most suitable lubricant based on factors such as temperature variations, load conditions, and operating environment.