From "How to Select a Motor Oil and Filter for Your Car or Truck"

Some synthetics, such as diesters and polyol esters have natural lubricity. Other synthetics, such as PAOs, offer no enhanced antiwear or friction-reducing properties by themselves. However, formulated motor oil generally consists of a blend of base oils plus additives that collectively impart friction and wear-reducing performance properties. PAO is the most common synthetic fluid used in motor oil formulations. It is frequently blended with a small concentration of an ester, such as a diester, to reduce seal shrinkage, improve additive solubility and provide better lubricity. However, it is worth noting that a mineral oil can be blended with esters and antiwear additives to achieve similar results.

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"I've heard that a lubricant's life is cut in half for every 10 degrees C (18 degrees F) increase in operating temperature. Is there any truth to this?"

Yes, this is generally true. The concept is based on the Arrhenius Rate Rule, named for 19th-century Swedish chemist Svante Arrhenius. Heat increases both the collision rate of molecules and the activation energy of the reaction. The higher activation energy helps overcome the barrier (or natural resistance) molecules have to chemical reactions.

With oils, the chemical reaction that typically causes base oil degradation and additive depletion is oxidation. The activation energy required to induce oxidation in oil is high compared to other chemical reactions. The presence of contaminants such as water and certain metal particles in the oil can considerably speed up the process, that is, increasing the activation rate. For most in-service mineral oils with typical contaminants, the activation energy for oxidation corresponds to a doubling for every 10 degrees C temperature increment.

Why does a refrigerator keep food from spoiling? Spoiling is a chemical reaction like oil oxidation but proceeds at a much higher rate constant. Compared to a refrigerator (4 degrees F), food will spoil at a rate 142 times faster at room temperature (77 degrees F). In other words, the speed of reaction in a freezer is much slower than at room temperature.

Jim Fitch, Noria Corporation

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