What Is Viscosity Index (VI)?
The viscosity index (VI) is a dimensionless number that expresses the relative behavior of an oil's viscosity with respect to temperature variation. In general, a higher VI indicates that the viscosity changes less as the temperature changes.
What does a higher VI provide?
In hydraulic systems and mobile equipment operating over a wide temperature range, more stable viscosity can provide advantages in terms of cold flow and hot film retention. However, VI alone does not define the overall performance of the oil.
Are VI and ISO VG the same thing?
No. ISO VG refers to the viscosity class of the oil at 40°C, while VI indicates how viscosity changes with temperature. Products with different VI values can be found within the same ISO VG class.
Effect of additives and base oils.
High VI can be achieved with naturally high VI base oils or viscosity index improving polymers. In applications requiring shear stability, the mechanical stability of the VI improver is also of particular importance.
How should it be used in product selection?
Minimum and maximum operating temperature, equipment speed, load, pump requirements, and OEM recommendations should be considered together. A higher VI (Visual Temperature) does not automatically mean a better product in every application.
Conclusion
VI is an important selection criterion, especially in applications with a wide temperature range. However, the viscosity class should not be evaluated independently of wear performance and equipment requirements.
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