What is the relationship between viscosity and temperature?
The viscosity of most fluids decreases as its temperature increases and vice verse. This occurs because as the liquid is heated, the force of attraction between the molecules, the cohesive forces, is reduced. This principle is very important to understanding differences between laboratory viscosity measurements and process viscosity measurements when they are performed at different temperature. A lubricants viscosity-temperature relationship is characterized by its VI (Viscosity Index).