Viscosity is the most important property of lubricant. In fact, a  lot of people just consider viscosity but disregard VI when choosing lubricant.

As viscosity determines film thickness and film strength which influences lubricity performance. Viscosity must be suitable for load, speed of lubricating components.

Viscosity of a lubricant is a parameter varies due to temperature changing, it means that viscosity is always a parameter at a specific temperature.

However, Machine Manufacturer's Engineers just base on load and speed of lubricating oil components to give required viscosity such as minimum, maximum and optimum viscosity for each component, don't take into account temperature although it influences viscosity.

In brief, when selecting lubricant, we know required viscocity, range of working temperature and know ... viscosity will change when temperature change. With hight VI lubricant, change rate of viscosity based on the temperature will be low. Conversely, with lower VI lubricant, change rate of viscosity based on the temperature will be higher.

Below table illustrates how viscosity of two lubricating oil which have the same ISO grade (150), different VI (150 & 95) change relative to a change in temperature.

If conditions are variable (e,g. loads, speeds, working temperatures), we should choose lubricating oil has the most appropriate optimum viscosity and high VI. In practice, defining the optimum viscosity requried for many machines are very difficult, so this is the reason why a high VI lubricant to be more selected and preferred.

Choosing lubricant has high VI when:

• It is difficult to know exactly the optimum viscosity
• Varying loads and speeds exist
• To boost energy efficency
• To boost oil service life
• To boost machine service life

Choosing lubricant has lower VI when:

• 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 achieve

Nowdays, there are a lot of website have online calculator ot help calculate the viscosity at a given temperature when the viscosity is known at two temperatures, or calculate viscosity at 40 oC, 100 oC, VI when two of three parameters are known.

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