What term describes the stability of fluid viscosity when it is heated?

The term that describes the stability of fluid viscosity when it is heated is known as the viscosity index. This index is a measure of how much a fluid's viscosity changes with temperature. Fluids with a high viscosity index retain their viscosity better when heated, meaning they don't thin out as much as those with a lower viscosity index. This is particularly important in applications where temperature fluctuations occur, as it ensures that the fluid continues to perform effectively under varying thermal conditions.

The other terms listed do not relate directly to the stability of viscosity with temperature. For example, the pour point refers to the lowest temperature at which a fluid will flow, while vapor pressure reflects the tendency of a fluid to evaporate. The oxidation rate is associated with the breakdown of materials in the presence of oxygen, which also does not pertain to viscosity stability specifically.