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Motor Oil Basics

Properties of Motor Oil

Most motor oils are made from a heavier, thicker petroleum hydrocarbon base stock derived from crude oil, with additives added as needed to improve the properties. One of the most important properties of motor oil in maintaining a lubricating film between moving parts is its viscosity. In layman's terms, the viscosity of a liquid can be thought of as its "thickness" or a quantity of resistance to flow. The viscosity must be high enough to maintain a satisfactory lubricating film, but low enough that the oil can flow around the engine parts satisfactorily to keep them well coated under all conditions. The viscosity index is a measure of how much the oil's viscosity changes as temperature changes. A higher viscosity index indicates the viscosity changes less with temperature than a lower viscosity index.

Another important property of motor oil is its pour point, which is indicative of the lowest temperature at which the oil could still be poured satisfactorily. The lower the pour point temperature of the oil, the more desirable the oil is when starting up at cold temperature.

Oil is largely composed of hydrocarbons, which can burn if ignited. Still another important property of motor oil is its flash point; the lowest temperature at which the oil gives off vapors which can ignite. It is, of course, dangerous for the oil to ignite and burn, so a high flash point is desirable. At a petroleum refinery, fractional distillation to separate crude oil fractions removes the volatile components, which more easily ignite, from the motor oil fraction; therefore, increasing the oil's flash point.

Another test done on oil is Total Base Number (TBN), which is a measurement of reserve alkalinity of an oil for neutralizing acids. The resulting quantity is determined as mg KOH/(gram of lubricant). Analogously, Total Acid Number (TAN) is the measurement of a lubricant’s acidity content. Other tests include zinc, phosphorus, or sulfur content, and testing for excessive foaming.

Most commercial oils have a minimal amount of zinc dialkyldithiophosphate as an anti-wear additive to protect contacting metal surfaces with zinc in case of metal to metal contact.

Motor Oil Viscosity and Horsepower

Using a high-viscosity motor oil could reduce your engine horsepower. It takes horsepower to push thick high-viscosity oil through the engine oil passages. Here's an example, you could lose 10 or more horsepower @ 5500 RPM in a Chevy 350 cid engine just by using a high-viscosity motor oil over a standard viscosity motor oil. In some cases your engine will not be any better protected using a high-viscosity motor oil over a lower-viscosity motor oil. Contact your favorite racing motor oil company for more information. Always follow your vehicle manufacturers motor oil recommendations.

Synthetic Oil and Synthetic Blends

Synthetic lubricants were first synthesized, or man-made, in significant quantities as replacements for mineral lubricants (and fuels) by German scientists in the late 1930s and early 1940s because of their lack of sufficient quantities of crude for their (primarily military) needs. A significant factor in its gain in popularity was the ability of synthetic-based lubricants to remain fluid in the sub-zero temperatures of the Eastern front in wintertime, temperatures which caused petroleum-based lubricants to solidify due to their higher wax content. The use of synthetic lubricants widened through the 1950s and 1960s due to a property at the other end of the temperature spectrum, the ability to lubricate aviation engines at temperatures that caused mineral-based lubricants to break down. In the mid 1970s, synthetic motor oils were formulated and commercially applied for the first time in automotive applications. The same SAE system for designating motor oil viscosity also applies to synthetic oils.

Instead of making motor oil with the conventional petroleum base, "true" synthetic oil base stocks are artificially synthesized. Synthetic oils are derived from either Group III mineral base oils, Group IV, or Group V non-mineral bases. True synthetics include classes of lubricants like synthetic esters as well as "others" like GTL (Methane Gas-to-Liquid) (Group V) and polyalpha-olefins (Group IV), although actual base oil content of finished blended motor oils is not a factor. Higher purity and therefore better property control theoretically means synthetic oil has good mechanical properties at extremes of high and low temperatures. The molecules are made large and "soft" enough to retain good viscosity at higher temperatures, yet branched molecular structures interfere with solidification and therefore allow flow at lower temperatures. Thus, although the viscosity still decreases as temperature increases, these synthetic motor oils have a much improved viscosity index over the traditional petroleum base. Their specially designed properties allow a wider temperature range at higher and lower temperatures and often include a lower pour point. With their improved viscosity index, true synthetic oils need little or no viscosity index improvers, which are the oil components most vulnerable to thermal and mechanical degradation as the oil ages, and thus they do not degrade as quickly as traditional motor oils.

Synthetic lubricants are designed for "long life" extended drain intervals, but most users rarely run them long enough to gain a cost-effective return. If a "synthetic" oil costs 2 to 3 times as much as a conventional oil, it would have to be used for 2 to 3 times longer than a conventional oil just to break even.

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