Viscosity Index (VI) is a scale used to measure how much an oil's viscosity, or resistance to flow, changes depending on its temperature. Generally speaking, the less it changes, across a range of temperatures, the better. The scale of viscosity index is numerical in nature, with zero being the most susceptible to changes in viscosity. It is a frequent basis for comparison in the automotive oil industry, and is often abbreviated as VI.
As oil heats up, its ability to provide effective lubrication diminishes. As this decreases, friction and heat increase, which can lead to mechanical failures. Therefore, the longer an oil can retain its optimum viscosity, the more effectively it will lubricate an engine and prevent damage. In this way, viscosity index can be a useful way of judging an oil's overall quality, and is an essential piece of information when selecting an oil for heavy-duty use involving wide variations in temperature.
The viscosity index of an oil is based on its measured viscosity at 100°F (40°C) and 210°F (100°C), which approximate the temperatures present in an engine when it is first turned on, and then after it has warmed up. The smaller the variation, the higher the score on the index. Since the development of viscosity index as a comparative tool, oil technology has improved and outgrown the original scale, which only ran to 100.
The best modern synthetic — that is, man-made — oils can rate over 400 on the scale, while petroleum-based oils can similarly exceed the 100 mark. For ease in comparison, the scale is sometimes split into several broad categories, with oils scoring below 35 classified as 'Low VI;' those scoring between 35 and 80 as 'Medium VI;' oils between 80 and 110 rated 'High VI;' and those above 110 classified as 'Very High VI.'
There are drawbacks to a high viscosity index, however. To achieve very high VI scores, oil manufacturers typically inject additives specifically designed to resist the effects of temperature change. There is a limit to how much of these additives can be added, without impacting the other desirable properties of oil.
Perhaps more importantly, additives tend to burn off under pressure. This can leave oil unable to resist thinning, and ultimately incapable of protecting the engine at high temperatures. As a result, viscosity index should not be the sole deciding factor when choosing which oil to use, though it remains a highly useful piece of data.