As long ago as 1911 the Society of Automotive Engineers recognized a need for standardizing grades of oil for automobile engines. Today the ratings are provided in collaboration with the ASTM and the American Petroleum Institute (API).
In the United States, engine oil ratings are indicated by a trademarked circular design on the package. The mark contains three pieces of information. In the inner circle is a viscosity grade. In the upper part of the outer circle is an API Service Classification, and the bottom of the outer circle may contain an energy conservation rating.
The package may also have a starburst symbol. The presence of this symbol indicates that the oil meets standards set by the International Lubricant Standardization and Approval Committee, whose members represent American and Japanese automobile manufacturers.
The purpose of the viscosity rating is to assure that, in the climate in which the engine is operated, the oil will always be able to flow through the engine's lubrication system. Eleven grades are defined. Six of them end in the letter “W,” beginning with “0W” and proceeding in steps of 5 to “25W.” The W stands for winter. The remaining five grades go from “20” to “60” in steps of 10, without the W. In general, higher numbers mean higher viscosities.
The viscosity of the oil is tested at 100°C. Its viscosity must be greater than a minimum that is specified for each grade. For the grades without a W, a maximum viscosity is also specified. This test basically ensures the oil will perform in a warm, running engine.
For grades ending in W, in addition to the minimum viscosity at 100°C test, a low temperature test is required. The temperature depends on the grade; for the 0W grade it is 35°C , for example. The purpose of the low temperature test is to ensure that if the oil's viscosity is low enough to permit cranking, it will also be low enough to be pumped through the engine. (Those processes involve different kinds of viscosity, so the cranking viscosity is measured in centipoises and the pumpability viscosity in centistokes.) The test temperatures rise by 5°C for each grade; oils with the lower numbers are intended for climates with colder winters. However, to select a viscosity grade for a particular car, consult the owner's manual and not just an outdoor thermometer.
Multiviscosity oils, such as “20W–50,” must satisfy both the low temperature cranking and pumping test for the grade indicated first, and the 100°C test for the second grade. Such an oil would also pass the tests for all the grades in between.
The original grades were “Viscosity Numbers”, and were based only on viscosity measured in Saybolt universal seconds. An oil whose visosity fell outside the specified ranges was placed in the next lower category.
|Viscosity Number||Saybolt universal seconds|
|at 130°F||at 210°F|
In 1947, three service classifications were added:
Both engines and the technology of lubrication became increasingly more sophisticated, so these classifications were replaced in 1952 with:
After a number of revisions, those classifications were replaced by an entirely new, more easily extended system in 1983.
For gasoline engines, service classifications start with SA and proceed through (so far) SJ. Classifications SA through SF are considered obsolete, except that SE and SF are needed for certain smog systems which can be poisoned by additives present in later classifications. SA has no requirements at all; while the others are for earlier generations of engine (SC, 1964; SD, 1968; SE, 1972; SF, 1980; SG, 1989; SH, ; SJ, 1996). SJ is suited to a 1996 engine.
A similar set of classifications covers diesel engines. CA, CB, and CC are obsolete; CD, CD-II, and CE are in current use.
If present, the marking will be either “EnergyConserving” or “EnergyConserving II.” To qualify as “EnergyConserving,” an engine oil must improve fuel economy in laboratory tests by at least 1.5% when compared with a reference oil. If the improvement is 2.7% or more, the oil can be labeled “EnergyConserving II.” These ratings don't guarantee that switching to “EnergyConserving” oil will improve mileage by 1.5% in a particular car, because of the large number of other factors at work.
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Last revised: 11 October 2013.