Viscosity Index (and why it matters)
Lubricants are given an ISO Grade based on their viscosity. The lower the ISO grade, the thinner and runnier the oil. However, the ISO Grade is only half of the story. ISO Grades are determined by the viscosity at one temperature: 40ºC (104ºF). Viscosity decreases as temperature increases, so as your equipment heats up, the oil in it will thin down and flow more easily. This property is true for all oils, but its impact is determined by the Viscosity Index.
The Viscosity Index is a measure for how much the oil viscosity changes as a function of temperature. Contrary to what you might expect, a higher Viscosity Index means a smaller change in viscosity. When an oil maintains its viscosity over a wider temperature range it can deliver more consistent performance and extend the operating envelope of the equipment.
In most equipment the lubricating oil is designed to get between two moving surfaces and provide a cushion. The cushion of lubricant offers several advantages, most notably: reducing wear, dissipating heat, and smoother operation. Thick oils (and semi-solid greases) do an excellent job of remaining in place and reducing wear (so less heat is generated), but don’t dissipate heat as well. Thinner oils flow more readily and dissipate heat, but might run off and leave some of the contact area under-lubricated.
In normal operation, these effects work synergistically to keep everything running smooth. The lubricating oil works as a cushion, heats up slightly, and then flows away.
However, when temperatures start to run too high, the “self-correction” mechanism breaks down. The oil cushion gets too thin and is no longer able to effectively reduce friction. Increased friction increases heat generation even further, driving up the temperature of the oil and lowering its viscosity even more. Eventually this will lead to increased wear and tear on the machine, ultimately reducing its life expectancy.
The best prevention against this, and the way to get the best performance over a wide operating temperature range is the same: use oils with a higher Viscosity Index. These oils retain their viscosity over a wider range so changes in temperature have less of an impact on their performance. If you're unsure, you can always calculate the Viscosity Index of your oils, but it's usually listed on the Technical Data Sheet (TDS) or Product Data Sheet (PDS) offered by major suppliers.