Rick's Free Auto Repair Advice

Viscosity Index

What is viscosity index?

Viscosity index is a value that represents how much the viscosity (thickness/flow) of a lubricating fluid, motor oil, transmission fluid, power steering fluid or gear oil changes with temperatures between 40°C and 100°C (ASTM D2270 test routine).

All lubricating oil thins as temperature increases. In other words, it thins out. As temperature decreases, viscosity increases. To combat thinning at higher temperatures, manufacturers add viscosity improvers.  So the total change in viscosity over a given temperature range is its viscosity index of VI. The higher the VI, the less the lubricating fluid/oil changes viscosity over the given temperature range. The lower the number, the more the fluid/oil will change viscosity.

Why is a higher viscosity index better?

The best oils are the ones with the highest VI because they remain stable with the least amount of viscosity variation over a wide temperature range. So they provide more consistent flow and lubrication to the moving parts.

A lubricating fluid or oil with a higher VI means the oil is more stable across a broad temperature range than the same “weight” oil with a lower VI specification.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat


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