The VI was originally measured on a scale from 0 to 100; however, advancements in lubrication science have led to the development of oils with much higher VIs.
[1] The viscosity of a lubricant is closely related to its ability to reduce friction in solid body contacts.
Generally, the least viscous lubricant which still forces the two moving surfaces apart to achieve "fluid bearing" conditions is desired.
If the lubricant is too viscous, it will require a large amount of energy to move (as in honey); if it is too thin, the surfaces will come in contact and friction will increase.
Thus the multigrade oils are recommended for use based on the ambient temperature ranges of the season or environment.
[3] VI improving additives and higher-quality base oils are widely used nowadays, which increase the VIs attainable beyond the value of 100.