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VISCOSITY

 Viscosity is the measure of how thick an oil is. This is the most important property for an engine. An oil with too low a viscosity can shear and loose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm. The weights given on oils are arbitrary numbers assigned by the S.A.E. (Society of Automotive Engineers). These numbers correspond to "real" viscosity, as measured by several accepted techniques. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50 by the S.A.E. The W means the oil meets specifications for viscosity at 0 F and is therefore suitable for Winter use.

Multi-viscosity oils work like this: Polymers are added to a light base(5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-viscosity oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot. Multi-viscosity oils are one of the great improvements in oils, but they should be chosen wisely. Always use a multi-grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter. In the winter base your decision on the lowest temperature you will encounter, in the summer, the highest temperature you expect.

Motor Oil Quality Designations
Performance and Technical information about engine motor oil.

If you own a new, or fairly new vehicle, to keep your warranty valid you must use an oil that meets the manufacturer's specifications. Virtually all well known motor oils  exceed warranty requirements for US, European, and Japanese cars. But what about the other letters and symbols you find on the bottle, such as API, SJ, SH, CF, CG-4? Or SAE 5W-30?

API stands for the American Petroleum Institute, which publish standards high quality oils must meet.

SJ was adopted in 1996 and covers oils used in gasoline engines in current and earlier passenger cars, sport utility vehicles, vans and light trucks operated under vehicle manufacturers' recommended maintenance procedures. SJ oils may be used where SH and earlier categories have been recommended.

SH applies to gasoline engines in cars, vans and light trucks and indicates that the oil provides improved control of engine deposits, oil oxidization, and engine wear compared to earlier oils. It also provides protection against rust and corrosion.

CF was adopted in 1994 for use in indirect injected off-road diesel engines that use a broad range of fuels, including those with a high sulfur content. It offers effective control of piston deposits, wear and corrosion of the copper-coated bearings used in this type of engine.

SAE stands for Society of Automotive Engineers. Motor oils have SAE grades, or numbers that indicate viscosity. In other words, the SAE numbers tell you the "thickness" of the oil. The lower the number, the "thinner" the oil.

W signifies its winter rating, showing that it will perform well in particularly cold weather.

When you buy oil, or have it changed, be sure not only to pick the right API rating for your engine, but ask for the SAE viscosity recommended in your owner's manual. Normally, manufacturers recommend multi-viscosity grades, which are suitable for use over a wide temperature range.