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Viscosity index (VI) measures an oil’s resistance to flow at different temperatures. The higher the VI, the better the oil lubricates an engine. VI is based on viscosity measurements at 100°F and 210°F. Modern synthetic oils can score over 400, but high VI oils may contain additives that burn under pressure. VI should not be the only factor in choosing oil.
Viscosity index (VI) is a scale used to measure how much an oil’s viscosity, or resistance to flow, changes depending on its temperature. In general, the less it changes, over a range of temperatures, the better. The viscosity index scale is numerical in nature, with zero being the most susceptible to changes in viscosity. It is a frequent basis of comparison in the automotive oil industry and is often abbreviated as VI.
As the oil warms up, its ability to provide effective lubrication decreases. When this decreases, friction and heat increase which can lead to mechanical failure. Therefore, the longer an oil can maintain its optimum viscosity, the more effectively it will lubricate an engine and prevent damage. In this way, the viscosity index can be a useful way to judge the overall quality of an oil, and is essential information in selecting an oil for heavy-duty applications involving wide temperature variations.
The viscosity index of an oil is based on its viscosity measured at 100°F (40°C) and 210°F (100°C), which approximate the temperatures experienced in an engine when it is first started, and then after that it warmed up. The smaller the change, the higher the score on the index. Since the development of the viscosity index as a comparative tool, petroleum technology has improved and surpassed the original scale, which only went as high as 100.
The best modern synthetic, i.e. man-made, oils can rate over 400 on the scale, while petroleum-based oils can similarly top the 100 mark. For ease of comparison, the scale is sometimes broken down into several broad categories, with oils scoring less than 35 classified as ‘Low VI;’ those scoring between 35 and 80 as ‘Medium VI;’ oils between 80 and 110 classified ‘High VI;’ and those above 110 classified as “Very High VI”.
However, there are drawbacks to a high viscosity index. 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 affecting the other desirable properties of the oil.
Perhaps more importantly, additives tend to burn under pressure. This can leave the oil unable to resist thinning and ultimately unable to protect the engine at high temperatures. Consequently, the viscosity index should not be the only deciding factor in choosing which oil to use, although it remains a very useful figure.
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