The volume of a given oil mass increases with temperature, therefore, its density decreases. The degree of expansion is expressed as the coefficient of thermal expansion. Thermal expansion is useful to determine the size of a container needed when the oil will be heated. Inexperienced people often have an oil overflow because of a surprising amount of thermal expansion.

In hydrodynamic lubrication, the thermal expansion of the oil in the clearance of a bearing increases the hydraulic pressure. Some researchers discuss the "thermal wedge" mechanism of film formation and apply it to parallel sliding surfaces, especially flat, non tilting, thrust bearings.

The coefficient of thermal expansion is the ratio of the relative change of volume to a change in temperature. Thermal expansion is expressed as the ratio of volume change to the initial volume after heating 1 degree C. Therefore, the unit is reciprocal degree C, or degree C-1. The values of the coefficient of thermal expansion for mineral oil are near 6.4 X 10-4 °C-1.

Thermal expansion (or contraction) determinations require the measurement of the volume of a given mass of oil at various temperatures. The sample is placed in a graduated cylinder and the volume is observed as the temperature is either increased or decreased. A simplified method of calculating the thermal expansion of petroleum products can be found in ASTM D 1250, Petroleum Measurement Tables, "Volume Corrections Factors".