Surface tension is the surface energy between a liquid and its own vapor, or air, or a metal surface. The word tension comes from the force that resists any attempt to increase the surface area. Surface tension is thought to be a factor in the ability of an oil to "wet" a surface, in emulsion stability, and in the stability of dispersed solids. However, "wetting" has been found to be a complex phenomenon involving oleophobic and oleophilic films on the metal surface. Some additives markedly change surface tension. An example is water containing soap for the formation of bubbles. Silicone is added to mineral oils to reduce surface tension and as result, foaming characteristics may change.

Surface tension of oils on metals can be compared by placing a small drop on a clean metal surface and observing if the drop has a high contact angle (high surface tension) or very low contact angle (spreads out, low surface tension). Surface tension between an oil and a metal surface is measured by using the Nouy ring method. The method involves placing a clean, platinum wire ring on the surface of the oil, where the force required to pull the ring away from the surface is measured.

Interfacial tension exists between two liquid layers and is measured by ASTM D-971. The method uses a tensiometer and a platinum ring. The ring is lowered into a beaker of water and oil. It is then brought up to the water-oil interface where the actual measurement takes place.

The force required to pull the ring through the interface is measured by the tensiometer and considered to be the interfacial tension of the oil. The value for mineral oil varies from 30 to 35 dynes cm-1.