Today, the kinematic viscosity in mm²/s is used for the VI calculation. Initially, the viscosity was measured in Saybolt Universal Seconds (SUS). Today, temperatures of 40 ☌ and 100 ☌ are common. The Fahrenheit temperature scale is still frequently used in the Anglo-American area. In the early VI systems, the temperatures for viscosity measurement were 100 ☏ and 210 ☏, corresponding to 37.78 ☌ and 98.89 ☌. To increase the VI to values higher than 100, new base oil types and special additives were developed later on. In the middle, a VI of around 50 would be assigned.
If the oil was similar to the paraffinic oil, a VI of 100 was assigned if it was similar to the naphthenic oil, a VI of 0 was assigned. Lube oils were then compared to these benchmarks. Their VI was indicated with 0 – this was the worst possible VI. Oils with significant viscosity change (LVI-oils, refined from Texas gulf crudes, naphthenic oil) represented the low end. Their VI was indicated with 100, which represented the best VI. Oils with low temperature-dependent viscosity change (HVI-oils, refined from Pennsylvania crudes, paraffinic oil) were on the high end of the scale.
For the VI scale, two limit points were set.
At this time, no multi-grade oils and no synthetic oils were available. For this purpose, the viscosity index 12 for base stocks and lube oils was developed by Dean and Davis from Standard Oil in the year 1929. For any oil used under changing temperature conditions, it is important to know the change of viscosity in relation to the change of temperature. Temperature is the most influential parameter for the viscosity change of oil. The viscosity of lube oil does not give any relevant information unless the temperature, at which it was measured, is known.
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