American engineers noticed that oil from Texas Gulf crude was much thicker at sub-zero temperatures compared to oil from Pennsylvania crude during World War II.
This observation came to light when the U.S. military required large quantities of oil for use in various theaters of the war, including cold environments. Engineers and researchers discovered that the paraffin-based Texas crude became significantly more viscous at low temperatures, posing challenges for its use in cold climates. This finding highlighted the differences in the chemical compositions of crude oils from different regions, with Pennsylvania crude being more suitable for low-temperature applications due to its lower wax content and better flow properties at sub-zero temperatures.
To measure this, they developed the Viscosity Index (VI) in 1929.
The VI uses Pennsylvania crude (paraffinic) as a benchmark with a VI of 100, representing low viscosity change with temperature. Texas Gulf crude (naphthenic) was set at 0. Oils similar to Pennsylvania crude received higher VIs, indicating more stable viscosity across temperatures.
The VI is a numerical, unitless measure of how much an oil’s viscosity changes with temperature, with measurements taken at 40°C and 100°C. Higher VI values indicate less change in viscosity with temperature, which is more desirable.