API Engine Oil Classifications

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The American Petroleum Institute (API) has established a classification system for the designation of gasoline and diesel engine oils, which reflects the quality, performance, and suitability of the oils for various engines. These classifications, or categories as sometimes referred, have no bearing on oil viscosity, whose limits are set by the Society of Automotive Engineers (SAE) and designated in SAE J300. Contrary to the belief of many, the SAE grade only defines oil viscosity and has absolutely nothing to do with oil quality. To specify engine oil, both the API service designation and the SAE viscosity grade are required.

In the United States, API also administers the licensing and certification of engine oils through a classification system that reflects the warranty, maintenance and lubrication requirements of the automotive industry. Through this system, API has standardized the labeling of engine oils by adopting the "donut” logo, (see Figure 1) which tells the user the oil's viscosity grade, engine service classification and any energy conserving capabilities.

The energy conserving capabilities were established in 1994. The products are currently restricted to multigrades of 20 and 30 weight motor oils (typically 5W-20, 5W-30 and 10W-30). The energy conserving grades will show both the starburst (see Figure 2) on the front label and the API donut showing energy conserving on the back label. Energy conserving grades of SM motor oil meet API GF-4 requirements.

The engine and vehicle manufacturers and technical societies establish engine oil performance requirements, test methods and limits for the various classifications.

These classifications are arranged into two different groups, one for automotive gasoline engine service and the other for commercial diesel engine service. The former is listed in "S" or "Service" categories (presently SA through SM) and the later is listed in "C" or "Commercial" categories (presently CA through CI-4 PLUS).


Typically the automotive category will only show the “S” service category in the donut (see Figure 1) while the “C” category will have the C category followed by the S service category. Hence a donut for a diesel engine oil meeting both API CI-4 Plus (diesel) and API SM (gasoline) is shown in Figure 3.

(courtesy Fina)

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CJ-4 Service Description (courtesy API)