For instance, oil with a 5W-30 grade thickens less than oil with a 10W-30 grade in cold weather. The numbers after the “XW” indicate viscosity at 100 degrees Celsius and represent the oil’s resistance to thinning at high temperatures. The lower the number, the less the oil thickens in cold weather. For example, a 10W-30 motor oil means the viscosity is at 10W when the engine is cold and 30 when the engine is hot. So what do the numbers mean? The number preceding the W designates the viscosity at a low temperature, and the number following represents viscosity when the engine is hot. This means a lubricant with a higher viscosity index is more desirable, because it provides a more stable lubricating film over a wider temperature range. Viscosity index diagram The slope of the lubricant with a high VI is more horizontal: the viscosity remains more stable across a wider temperature range.

Highly refined mineral oils have a VI of approximately 120. While the VI has no units of measure, it is well known that the viscosity index of a conventional mineral oil is between 95 to 100. Thus, the viscosity index measures the ability of an engine oil to resist becoming thinner at high temperatures.

The viscosity changes with temperature to ensure protection and operability under all circumstances. Oil viscosity index explained A higher viscosity implies a slower flow and a thicker fluid. What does a high viscosity index mean for an oil? The viscosity index is important in order to ensure, e.g., the best possible lubrication for machinery, as temperature changes occur during operation. It allows for a better comparison of the viscosity behavior of different oils based on temperature. The higher the VI, the more stable the viscosity remains over temperature fluctuations. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils. The viscosity index (VI) is an arbitrary, unit-less measure of a fluid’s change in viscosity relative to temperature change.

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