Question

W what is the viscosity index of the lubricant and its importance

Answer 1

The viscosity index (VI) is an arbitrary, unitless measure of the change of viscosity with temperature, mostly used to characterize the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the viscosity is affected by changes in temperature

Answer 2

It decreases (thins) with increasing temperature and increases (or thickens) with decreased temperature. The benefits of oils with a higher viscosity is that a general increase in viscosity at higher temperatures results in lower oil consumption and less wear, which is profitable for the environment.