Which property classifies the majority of oils?

Viscosity is a key property that classifies the majority of oils because it defines the thickness or flow characteristics of the oil. It indicates how easily the oil can flow at various temperatures. Understanding viscosity is crucial for selecting the correct oil for different applications, ensuring that it performs effectively under varying conditions. For example, an oil with a high viscosity is thicker and flows less easily compared to one with a low viscosity, which is thinner and flows more freely.

In the context of equipment maintenance, viscosity also affects lubrication effectiveness, heat transfer, and the ability of the oil to maintain a protective film between moving parts. Thus, it is the most relevant property to consider when classifying oils for specific uses. Other properties like temperature, color, and density can provide useful information but do not primarily categorize oils in the way that viscosity does.