ISO (International Organization for Standardization) is the standard that defines oil viscosity classifications. It provides a standardized grading system of lubricant viscosity called ISO VG (viscosity grade).
Viscosity index (VI), on the other hand, is a measure of the change in oil viscosity with temperature. It quantifies the change in oil viscosity at different operating temperatures. Oils with higher VI have less viscosity change with temperature, indicating better viscosity stability over a wider temperature range.
While ISO and viscosity index both relate to the viscosity of oil, they are not the same. The ISO VG system classifies oils based on their kinematic viscosity (measured in centistokes) at 40°C, while the Viscosity Index measures the change in viscosity with temperature.
When selecting a lubricant, it is important to consider the ISO VG classification viscosity grade and viscosity index. The ISO VG grade indicates the oil’s viscosity at a reference temperature, while the viscosity index indicates how the oil’s viscosity changes with temperature.
The combination of ISO VG grade and viscosity index helps ensure that the lubricant maintains the proper viscosity and provides optimal protection across the entire operating temperature range. Oil manufacturers carefully select the appropriate ISO VG grade and formulate oils with the required viscosity index to meet the requirements of specific applications and operating conditions.
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