How is viscosity index calculated for unknown oil?

The viscosity index of unknown lubricating oil is determined by comparing it with two standard oils. The two types of the standard are paraffinic base oil Pennsylvanian oil or Gulf oil or low viscosity standard having a viscosity index of zero( V.I=0).

The viscosities of the unknown lubricating oil sample are determined at 100 0F and 210 0F by any standard viscometers like redwood, Engler or saybolt. If the difference between the values is low then the oil is considered good lubricant else not.

From the list of high viscosity standard, the oil which has same viscosity at 210⁰ as the under test is selected and its corresponding viscosity at 100⁰F is noted as H.

Similarly, from the list of low selected standards, the oil which same viscosity at 210F as the oil under test is selected and its corresponding viscosity at 100⁰F is noted as L

The viscosity index (V.I) of lubricating oil is calculated using the following formula-
V.I= (L-U)/(L-H)×100
Where
V.I→ viscosity index of lubricating oil.
L→ viscosity of naphthenic base oil or Gulf oil or low viscosity standard at 100⁰F
H→viscosity of paraffinic base oil or Pennsylvanian oil or high viscosity standard at 100⁰ F
U→ viscosity of unknown oil sample under test at 100⁰F