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: