The ABC’s of OIL

Learning about designations, viscosities and change intervals isn’t difficult and can net longer service life

Ever wonder what all those numbers, letters and symbols on an oil container mean? If not, don’t worry — you’re not alone. But, if you are a dedicated car/truck/RV nerd, you should learn, because the science and technology behind contemporary oil formulations continues to change. Plus, understanding the oils on the market can help you get the most out of your engine in the long run.

The American Petroleum Institute (API) is the organization that represents all segments of America’s oil and natural gas industry, and just about everything you need to know about an oil is on the API “donut” on the back of the container.

For example, at the top of the donut, you’ll see the words “API Service,” followed by two or more letters. Letters beginning with the letter S (such as SN, SM, SL, SJ) are service categories designed for gasoline-burning engines. Letters beginning with C (such as CK-4 CJ-4, CI-4 and CH-4) are commercial categories designed for diesel applications.

In the center of the donut are the numbers which indicate the viscosity grade of the oil. Viscosity is a measure of an oil’s thickness — its ability to flow at certain temperatures — typically expressed in numbered grades ranging from 0 (thinnest) to 50 (thickest). Originally established by the Society of Automotive Engineers (SAE), an oil’s viscosity was once a single grade, but that changed when the SAE added winter grade designations, indicated by a “W” after the viscosity grade. The evolution of motor oil took another big step when advances in petrochemical engineering led to the development of viscosity enhancers, which made it possible for a single oil to serve double-duty in both low and high temperatures. These became known as “multi-grade” oils and are the ones we utilize today.

Generally speaking, it is recommended that you stick with the oil grade recommended by the manufacturer. However, bear in mind that the manufacturer’s recommendation is based on a new or as-new engine operated in a typical environment; a high-mileage engine, or one operated in extreme heat or cold, may be better suited to a different oil grade. Moreover, oils have a temperature operating range, so if you’re in a jam and need to add a quart or two of oil to your engine but your grade isn’t available, you’ll be fine to select a different grade (for example, 10W-30 or 10W-40 instead of 5W-30). In fact, there are some oils designed for diesel engine service that might also meet gasoline engine service standards. These “multiple performance level” oils can be identified by the “C” category first, followed by the “S” category (example: “API Service CJ-4, CI-4/SN”).

When using diesel fuels with sulfur content greater than 15 ppm (0.0015% by weight), refer to engine manufacturer recommendations for oil and/or service intervals. Many of these oils are formulated for use in applications with diesel fuels ranging in sulfur content up to 500 ppm (0.05% by weight). However, use of these oils with greater than 15 ppm (0.0015% by weight) sulfur fuel may impact exhaust after-treatment system durability and/or oil drain intervals.

So, what’s in the letters? These current designations from API will help give you a clue. We’ve omitted obsolete designations because they are not only not available, using them in a newer engine could actually do harm. On the other hand, using a current oil of the correct designation and viscosity in an older engine will not have adverse affects.

Gasoline Engines

SP: Introduced in May 2020, SP is designed to provide protection against low-speed pre-ignition (LSPI), timing-chain-wear, improve high-temperature-deposit protection for pistons and turbochargers, and provide more stringent sludge and varnish control. API SP with Resource Conserving matches ILSAC GF-6A by combining API SP performance with improved fuel economy, emission-control-system protection and protection of engines operating on fuels containing ethanol up to E85.

SN: For 2020 and older automotive engines.

SM: For 2010 and older automotive engines.

SL: For 2004 and older automotive engines.

SJ: For 2001 and older automotive engines.

Diesel Engines

CK-4: API Service Category CK-4 describes oils for use in high-speed four-stroke-cycle diesel engines designed to meet 2017 model year on-highway and Tier 4 non-road exhaust-emission standards as well as for previous model year diesel engines. These oils are especially effective at sustaining emission-control-system durability where particulate filters and other advanced aftertreatment systems are used. API CK-4 oils exceed the performance criteria of API CJ-4, CI-4 with CI-4 PLUS, and CH-4 and can effectively lubricate engines calling for those API Service Categories.

CJ-4: Introduced in 2010, CJ-4 is for high-speed, four-stroke engines designed to meet 2010 model year on-highway and Tier 4 non-road exhaust-emission standards as well as for previous model year diesel engines. API CJ-4 oils exceed the performance criteria of API CI-4 with CI-4 PLUS, CH-4, CG-4 and CF-4 and can effectively lubricate engines calling for those API Service Categories.

CI-4: Introduced in 2002. For high-speed, four-stroke engines designed to meet 2004 exhaust emission standards implemented in 2002. CI-4 oils are formulated to sustain engine durability where exhaust gas recirculation (EGR) is used and are intended for use with diesel fuels ranging in sulfur content up to 0.5% weight. Can be used in place of CD, CE, CF-4, CG-4, and CH-4 oils.

CH-4: Introduced in 1998. For high-speed, four-stroke engines designed to meet 1998 exhaust emission standards. CH-4 oils are specifically compounded for use with diesel fuels ranging in sulfur content up to 0.5% weight. Can be used in place of CD, CE, CF-4, and CG-4 oils.

FA-4: API Service Category FA-4 describes certain XW-30 oils specifically formulated for use in select high-speed four-stroke-cycle diesel engines designed to meet 2017 model year on-highway greenhouse gas (GHG) emission standards. Refer to individual engine manufacturer recommendations regarding compatibility with API FA-4 oils. These oils are especially effective at sustaining emission-control-system durability where particulate filters and other advanced aftertreatment systems are used. API FA-4 oils are not interchangeable or backward compatible with API CK-4, CJ-4, CI-4 with CI-4 PLUS, CI-4, and CH-4 oils. Refer to engine manufacturer recommendations to determine if API FA-4 oils are suitable for use.