VI - The viscosity index
Viscosity is the most important physical property of a lubricating oil of all. It is of decisive importance for the ability of a lubricant to form a stable lubricating film. Read more, for example, in ÖlChecker Summer 2012.
In 1929, American scientists Dean and Davis discovered that different oils can behave completely differently at different temperatures. And this despite the fact that they have the same viscosity at 40 °C! While some of the oils were still relatively thin and flowed well even at a low temperature, the others had already solidified into a solid mass - disastrous for a lubricating oil.
To further investigate their discovery, Dean and Davis systematically examined a variety of lubricating oils on the market regarding their viscosity-temperature behavior. In the process, they established a set of rules that is still valid today. The viscosity index “VI” is calculated using the viscosities measured at two different temperatures (40 °C and 100 °C). With the help of this viscosity index, it is now much easier to compare the temperature behavior of different oils.
The higher the viscosity index, the less its viscosity changes at different temperatures. This is particularly advantageous when lubricating oils are used at both low and high temperatures.
Typical examples of oils with optimized viscosity-temperature behavior are modern multigrade engine oils such as SAE10W-40, 5W-30, 5W-40, 0W-40, 0W-20, 0W-16 or 0W-8. Such multigrade oils usually have a much higher viscosity index than, for example, monograde engine oils based purely on mineral oil, thanks to the addition of additives or the use of base oils based on esters and PAOs (polyalphaolefins).
Here is an overview of common oils and their typical VI:
| Oil type | Viscosity index |
|---|---|
| Gear and hydraulic oils | 95 - 105 |
| Multigrade engine oil SAE 15W-40 | 130 - 150 |
| Multigrade hydraulic oils | 130 - 160 |
| Multigrade engine oil SAE 5W-40 | 160 - 180 |
| Gear oil SAE 75W-90 | 160 - 190 |
| Bio-hydraulic oil HEES 46 | 140 - 200 |
Pure mineral lubricating oils usually have a VI of about 95. The viscosity index of synthetic oils or mineral oils subsequently thickened with special additives, on the other hand, is well over 130.
Bio-hydraulic oils have a much higher natural VI than mineral oils and are thus clearly superior to them, at least in this aspect.
The viscosity index VI is a parameter calculated using the kinematic viscosity measured at 40 °C and 100 °C. The calculation method described in ISO 2909 was already developed in the late 1920s. Paraffin-based mineral oils, which at that time showed the best viscosity-temperature behavior, i.e. which changed little with temperature, were assigned a viscosity index of 100. Oils that became significantly thinner with increasing temperatures were given the VI of 0. Synthetic oils or multigrade oils available today, i.e. oils with viscosity-changing additives (VI improvers), have a viscosity index well above 100. Oils with polymer-containing additives do not follow Newton's flow laws or do so only inaccurately. However, even "normal" lubricating oils become non-Newtonian fluids at the latest below the pour point, at which turbidity begins due to kerosene precipitation. Each oil has an individual viscosity-temperature behavior. With the help of the viscosity index, it is easier to compare the temperature behavior of different oils. The higher the viscosity index, the less its viscosity changes at different temperatures.
