Viscosity Index (VI) is a data point reference used on our technical data sheets. The VI data point or scale was originally devised by the Society of Automotive Engineers (SAE) in the USA. VI is a unit-less number that gives an indication of how a lubricating oil (or base oil if used to create a grease) viscosity alters with temperature change. The lower the VI number, the more that the viscosity of an oil changes when temperature changes; conversely the higher the VI number, the less an oil will change in viscosity with temperature change. Originally the VI scale was only 0 to 100 but with advancements in chemistry and lubrication technology, the scale has increased.
Considering VI’s via the graph opposite makes things easier to understand. The X and Y axis are logarithmic scales of viscosity and temperature, the orange line shows a higher VI as flatter line, the blue line is a lower VI. Some VI brackets of the base oils we use, including mineral oil to help the reader:
- Mineral Oil: 80-100
- PAO: 125-145
- Ester: 120-150
- PFPE: 100-350
- Silicone: 300-800
So for example, a mineral oil will change viscosity considerably with temperature change; think of how runny an engine oil becomes when heated. Synthetic oils have higher VI’s than mineral oils. Different Perfluoropolyether (PFPE) oils have a wide variety possible VI numbers and as shown, Silicone oils have the highest VI numbers, making those the oils alter viscosity the least with temperature change.
VI is normally only important if you are benchmarking one product against another and when considering the use of a grease, remember that VI only relates to the base oil used to make the grease, not the finished grease. Also, when we create a grease there is a possibility of using a VI modifier additive to help with improving a finished grease temperature stability.
What does Viscosity Index look like on a data sheet? Two examples Below:
Finally, if interested!….how is the seemingly arbitrary VI number generated? The Viscosity Index number is generated by an equation that involves using the kinematic viscosity of an oil at 40°C and 100°C. The equation used to determine VI is VI= 100((L-U)/(L-H)), where L is the oil having VI 0 at 40°C, U is a oil kinematic viscosity at 40°C, H is the oil having VI 100 at 40°C.
If you’ve read this far then we recommend two other associated articles on this website: