The viscosity of an oil is affected most by?

The viscosity of an oil is primarily influenced by temperature because viscosity is a measure of a fluid's resistance to flow and deformation. As temperature increases, the thermal energy causes the molecules in the oil to move more freely. This increased motion reduces the interactions and friction between the molecules, resulting in lower viscosity. Conversely, when the temperature decreases, the molecules move less vigorously, increasing the oil's viscosity and making it thicker.

Other factors, while they can also have some impact, do not affect viscosity to the same extent as temperature does. For example, contamination can lead to changes in viscosity, but these changes are typically minor compared to the effects observed when temperature varies. Resistance to corrosion relates more to the chemical stability of the oil than to its physical flow characteristics. While pressure can alter viscosity in some fluids, the effect of temperature is considerably more pronounced in most oils. Thus, temperature is the most significant factor influencing viscosity.