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Oil Performance -- Shear vs. Film Strength

Oil performs three functions in an engine, all equally important to performance: -- Placing a physical barrier between moving metal parts -- Cooling the engine, and keep the tolerances of the moving parts within spec -- Reducing friction, which has the result of reducing drag and increasing horsepower The tradeoffs required to accomplish all three tasks at the same time are the problem.

For instance, you might say that water would be a preferred lubricant because of its heat conductivity and its flow speed. However, at the operating temperatures of even old engines and certainly at the temperatures of new high performance engines, water vaporized before it could accomplish any cooling from circulation.

You might also decide you'd want a physical barrier that remained on the pistons after cool-down, so that in those first critical start-up seconds the engine parts were protected. But try the experiment of axle grease as engine lubricant and see what kind of performance you get. Obviously the drag on the parts would be catastrophic.

So you're left seeking a material that will have appropriate film strength, maintain its integrity at high temperature (100 C) and have flow characteristics that are as close to water as you can get without the vaporizing problem.

In the past, the buzzword in oil was "viscosity." The three oil tasks performed better or worse depending on the "viscosity" as measured in values of "film strength." For petroleum-based engine oils, "film strength" was a measurable and appropriate value. Enter the synthetics: Mobil 1, Texaco, Red Line and Royal Purple.

Synthetic lubricants benefit from being compounds with planned and predictable properties. In addition, the base stock can be supplemented with additives to improve specific properties. As the SMRP glossary entry for "synthetic lubricants" notes, many synthetic lubricants -- also called synlubes -- are derived wholly or primarily from petrochemicals; other synlube raw materials are derived from coal and oil shale, or are lipochemicals (from animal and vegetable oils). Synthetic lubricants may be superior to petroleum oils in specific performance areas. Many exhibit higher viscosity index (VI), better thermal stability and oxidation stability, and low volatility (which reduces oil consumption). Most synlubes offer longer service life and, in some cases, better biodegradability than conventional lubricants. Individual synthetic lubricants offer specific outstanding properties: phosphate esters, for example, are fire resistant, diesters have good oxidation stability and lubricity, and silicones offer exceptionally high VI. Polyalphaolefins are versatile lubricants with low pour points, and excellent thermal and oxidation stability; they have good compatibility with petroleum lubricants and most seals used with petroleum lubricants.

In testing, Royal Purple stands out as the oil with the best shear strength performance. In practical application, this means that you can run one grade lighter oil -- with less friction than with other synthetics -- without losing protection or endurance. Weight for weight, Royal Purple produces superior results in dyno testing. Comparing any other synthetic and one grade lighter Royal Purple the results are outstanding, in terms of protection, horsepower and endurance.

For additional technical information, see the accompanying University topic, Oil Technical Glossary.

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