"If you could perform a viscosity test at only one temperature, which would be the most beneficial — 40 or 100 degrees C?"
As you probably already know, viscosity is the most important property of an oil. It's the main criteria permitting the lubricant to perform as a separating barrier between contacting surfaces. Thus, it is essential that viscosity be at the top of nearly every oil analysis test slate. The testing standards for viscosity will typically either be for kinematic viscosity (resistance to flow and sheer by the forces of gravity) or absolute viscosity (the internal resistance to flow and sheer to the applied force, or dynamic viscosity).
Temperature is the most influential factor in measuring viscosity. As temperature goes up, viscosity goes down and vice versa. When you test for viscosity, you must normalize the temperature so you can compare among all other results without involving any correction calculations. It is standard for the temperature to be either 40 degrees C or 100 degrees C, and there are reasons for measuring viscosity at each of these temperatures.
Besides normalizing temperature for comparative reasons, it's also best to measure an oil's viscosity as close to what the operating temperature would be. Typically, 40 degrees C is not far from the average industrial machine operating temperature. Also, viscosity changes are more prominent at lower temperatures. Therefore, detecting any abnormalities, such as those influenced by water, fuel or oxidizing oil, would be easier at this lower temperature.
However, many machines operate at higher temperatures, such as diesel or gasoline engines. Thus, it is common for engine oil (and other lubricants under similar conditions) to be tested at 100 degrees C.
Another key lubricant property, viscosity index, describes the rate at which viscosity changes as the temperature changes. This is especially important with operating conditions that are expected to have temperature swings. In these situations, it will be necessary to know what viscosity should be expected at two temperature points in order to calculate the rate of change. In applications where multiple temperature-related objectives are involved, such as when multi-grade lubricants are selected, measuring at both of these temperatures may also be required.
In general, when selecting the temperature at which to test an oil's viscosity, be sure to ask the following questions: