What is it and what does it mean in practice?
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.
Frequently Asked Questions
Viscosity Index (VI) shows how much a lubricant’s viscosity changes with temperature. A higher VI means more stable viscosity across hot and cold conditions, helping maintain film thickness and reliability.
Read more about viscosity here.
Choose a high VI when your equipment sees meaningful temperature swings (outdoor assets, variable process temperatures, seasonal changes, start-stop duty). If temperatures are stable, selecting the correct base viscosity grade (ISO VG) usually matters more than maximising VI.
For a practical explanation of viscosity vs temperature and how VI supports stable performance, see: Viscosity of lubricants explained
ISO VG is the lubricant’s kinematic viscosity at 40°C (the “nominal thickness” at a reference temperature). VI describes how that viscosity changes between temperatures (how steeply it thins when temperature rises, and thickens when it drops). In practice: ISO VG helps you pick the baseline viscosity, VI tells you how stable that choice remains across real operating temperatures.
More about: Viscosity of lubricants: what is it and why is it important?
Yes. If you know the lubricant’s VI and its viscosity at a reference temperature (typically 40°C and/or 100°C), you can estimate its viscosity at another temperature (via standard calculation methods). In practice, this helps you verify whether the lubricant will still provide sufficient film at cold start and won’t become too thin at peak operating temperature.
Common signs include temperature-dependent issues: sluggish lubrication on cold starts, higher operating temperatures in warm conditions, and inconsistent noise or vibration as conditions change. If the response becomes “add more lubricant”, it can create new problems (overpressure, leakage, higher friction). For practical guidance on avoiding that trap, see: Prevent over-lubrication
