It is the rate of change of viscosity between 2 temperatures. The lower the Viscosity Index, the more the drop in viscosity as the oil warms up. The higher the VI value, the less the drop in viscosity as the oil warms up. Generally speaking, the less it changes, across a range of temperatures, the better.
The VI scale goes from 0 to 100, with 0 being the worst, and 100 being the best. New products are now better than when the scale was first made, so some new products have scores as high as 400.
As oil heats up, its ability to provide effective lubrication diminishes. As this decreases, friction and heat increase, which can lead to mechanical failures. Therefore, the longer an oil can retain its optimum viscosity, the more effectively it will lubricate an engine and prevent damage. In this way, viscosity index can be a useful way of judging an oil's overall quality, and is an essential piece of information when selecting an oil for heavy-duty use involving wide variations in temperature.
High or Low Viscosity Index ?
A lubricant may merit having a high VI for one or more of the following reasons:
- The optimum viscosity is not known
- Varying loads and speeds exist
- Varying ambient temperatures exist
- To boost energy efficiency
- To boost oil service life (lower average temperature)
- To boost machine service life (fewer repairs and downtime)
Cheaper, lower VI lubricants may make sense if
- Speeds and loads are constant
- Temperature is constant (constant ambient temperature or a heat exchanger is in use)
- The optimum viscosity at the operating temperature is known and is consistently achieved
An oil’s VI can also tell you useful information about a lubricant’s formulation, including the type and quality of base oils. For instance, highly refined and pure mineral oils will have correspondingly higher VIs. Certain additives, such as viscosity-index improvers and pour-point depressants, influence VI as well.