Understanding Viscosity Index
The fourth most important property of a lubricant.
by Evan Zabawski, CLS, OMA I 
Business Development/Reliability Specialist

Proper lubricant selection requires an understanding of the influence of temperature on viscosity.  Though most can appreciate that an increase in temperature causes a decrease in viscosity, we seldom consider that the rate of change is not only non-linear, but also that the rate of change is not equal between all lubricants.

The parameter that describes the rate of change is called the Viscosity Index, and it is an arbitrary measure represented by a unitless number.  The original scale was created in 1928 by petroleum chemists E.W. Dean and G.H.B. Davis, using Saybolt viscosity measurements of reference oils at both 100 °F and 212 °F.

The reference oils, which had the same viscosity at 212°F, were a naphthenic Coastal crude that exhibited the greatest change in viscosity and was assigned a Viscosity Index of 0, and a paraffinic Pennsylvania crude that exhibited the least change in viscosity and was assigned a Viscosity Index of 100.  This contemporary 0-100 scale could be used to qualify all products available at the time.

Once base stock refining techniques were improved, the ASTM D2270 calculation was devised to account for oils with a VI above 100.  With today’s technology it is rare to find a formulated product with a VI much below 100, and many multi-grades are achieving a VI close to 200.  In essence, we are now a full scale higher the original scale.  Oils have also been formulated to have a VI as low as -400, and the highest theoretical VI, one representing a flat viscosity-temperature curve, is approximately 700 – we’re not there yet.

This translates to having competing products in different grades able to achieve identical viscosities at the same operating temperature.  One example would be a high viscosity index ISO VG 220 oil compared to an average viscosity index ISO VG 320 oil.  The latter would likely be less expensive, but if used outdoors it could fall short at the colder ambient temperatures.

An everyday example to the average driver would be comparing SAE 0W oils to SAE 5W oils, with many drivers believing the 0W to be either a ‘winter only’ oil or simply too thin.  In fact, due to 0W oils’ high viscosity index, they can achieve a higher viscosity at the ring/cylinder interface than their higher grade cousins.  This is due in part to a greater dose of viscosity index improver additives, but also due to the fact that 0W oils are almost exclusively blended with synthetic or other high viscosity index base oils, while it is still possible to purchase some fairly low viscosity index 5W oils.

So even if the right viscosity with an appropriate VI is chosen, the next consideration would be the additive chemistry.  Now that becomes a myriad of fifth, sixth, seventh, etc… properties to consider when choosing a lubricant.  It goes to show that selecting lubricants is not always simple.