Engine Oil

All about oil viscosity and API classifications

Most people are familiar with oil's Viscosity Rating (10W40, for example). However, very few know that the "W" refers to "winter," not "weight." And most of us have no idea what the weight-rating numbers mean other than that the vehicle's manufacturer specifies a particular motor oil viscosity. This story deals strictly with viscosity and API Classifications.

Oil Duties

Inside an engine, oil is in a Catch-22 scenario: It has to seal rings and valves, but it also must reduce friction. In simple terms, oil has to accomplish two functions that have directly opposite requirements.

The viscosity of any oil changes with temperature. The higher the temperature, the lower the viscosity (the oil is thin). On the flipside, the lower the temperature the higher the viscosity (the oil is thick). Because of this, the Society of Automotive Engineers (SAE) has established a series of viscosity classifications that establish oil performance at 100 and 0 degrees Celsius (212 and 32 degrees Fahrenheit, respectively).

Highs and Lows

Low-viscosity oils flow better than high-viscosity types. The lighter-weight fluid is easier to pump and therefore circulates faster through the engine's various galleries. Low-viscosity oils also maintain a lower oil pressure, but the oil pump delivers a greater volume through the galleries than it would with thicker (higher-viscosity) oils. Heavier oils also tend to operate at higher temperatures because the oil pump has to work harder to force the lubricant through the system. Oil does not compress readily, so the added pressure increases the temperature. In the end, high-viscosity oils tend to maintain a higher oil pressure, but the pump tends to delivers a smaller volume of oil.


Multigrade oils typically begin as base oils, such as 10W. Then viscosity-index modifiers (polymers) are added in an effort to stabilize the viscosity. This allows a 10W40 oil to flow like a 10W at cold temperatures and a 40W at higher temperatures.

The multigrade oils' viscosity modifiers are long-chain molecules that lessen the change of viscosity with temperature variance. In the past, the polymer additives (used to thicken the oil) were sometimes susceptible to viscosity loss. Permanent viscosity loss occurred when high shear forces (such as the relationship between the main bearings and the crankshaft) actually break the polymer molecules into less-effective smaller pieces. On a similar note, temporary viscosity loss also occurred when the polymer molecules aligned themselves in order to create a path of least resistance.

Fortunately, today's additive packages have improved oil's shear-resistance. However, oils with the same rating from different manufacturers can exhibit different viscosity ratings in an operating engine, depending on the shear stability of their viscosity-modifying additives. Obviously, that's why there are different companies producing oil, and it's also why some brands offer superior capability over others.

For technoids, common single grade oils are defined as follows (stokes and centistrokes are measurements of viscosity):

  • SAE 30 is SAE 30 no matter what the "W" prefix number is: 0W, 5W or 10W. This viscosity in centistokes (cSt) @ 100 degrees C is with the minimum of 9.3 cSt and a maximum of 12.5 cSt.
  • SAE 40 is SAE 40 no matter what the "W" prefix number is: 5W, 10W, 15W or 20W. The viscosity @ 100 degrees C is within the minim of 12.5 cSt and a maximum of 16.3 cSt.
  • SAE 50 is SAE 50 no matter what the "W" prefix number is: 5W, 10W, 15W or 25W. The viscosity @ 100 degrees C is within the minimum of 16.3 cSt and a maximum of 21.9 cSt.
  • SAE 60 is SAE 60 no matter what the "W" prefix number is: 10W, 15W or 25W. The viscosity @ 100 degrees C is within the minimum of 21.9 cSt and a maximum of 26.1 cSt.

API Numbers

Shortly after WWII, the American Petroleum Institute (API) developed a system that established three basic types of engine oils: regular, premium and heavy-duty. Naturally, three oil classifications could never hope to cover all of the different applications ranging from conventional passenger cars to heavy-duty trucks. The API eventually realized that other variables had to be considered, such as the type of engine and its usage. In 1952, the API launched the service classifications system.

The API system revolves around two general classification: S for Service (typical "spark ignition" passenger cars and light trucks) and C for commercial applications (typical diesel equipment). The breakdown of "S" varieties is as follows:

  • SA: This is a plain mineral oil that doesn't contain additives common in today's high-tech lubricants. This oil was primarily used in the 1920s and is obsolete today.
  • SB: Lubes that contain anti-wear and oxidation inhibitors as well as corrosion inhibitors. This oil was primarily in use prior to 1964 and was created for vehicles that saw moderate conditions.
  • SC: This classification was originally recommended for use in 1964-67 vehicles. It contains additives that control rust, wear, corrosion and engine deposits. It too is now obsolete.
  • SD: SD lubes were recommended for use in 1968-70 vehicles as well as certain post-1970 passenger cars. This oil contains the same additive packages as the SC class and can be used in place of it. Now obsolete.
  • SE: This category was recommended for certain 1971 vehicles as well as most 1972 vehicles. This classification offers more protection than the SD group of lubricants and is suitable for severe-duty applications. This classification can be used in place of SD oils. Now obsolete.
  • SF: Recommended with 1980 and newer passenger vehicles. This oil has superior anti-wear properties and enhanced oxidation stability over SE lubricants. Now obsolete.
  • SG: The SG rating was introduced in 1989 and combined the performance properties of the commercial rating CC (lubricants designed for use in supercharged/turbocharged diesel applications in moderate to severe service). Now obsolete.
  • SH: Now obsolete, SH was designed for 1996 and older engines.
  • SJ: Current oil. Introduced in 1996, this rating is for all automotive engines presently in use from 2001 and earlier.
  • SL: Current oil. This API grade is for 2004 and older engines.
  • SM: Current oil. Introduced in late 2004, SM oils are engineered to provide improved oxidation resistance, improved deposit protection, better wear protection and better low temperature performance.

What if you have a passenger car that mandates a now obsolete API classification? According to the API: "For automotive gasoline engines, the latest engine oil service category includes the performance properties of each earlier category. If an automotive owner's manual calls for an API SJ or SL oil, an API SM oil will provide full protection."

Generally speaking, think of the API system as a modern blueprint for oil. In order to gain an API classification, oil manufacturers have to follow a set of limitations. This creates a few problems for oil companies, especially those who produce racing oils. Race oil must conform to viscosity-grade standards but not to those for chemical-additive composition and base-oil composition. That's why you'll find several brands of race (off-road) oil without API classifications.

About the Author

A true hands-on "gearhead," Wayne Scraba has a diverse background in both writing and motorsports. Over the past two and a half decades, Scraba toiled as a magazine editor, technical editor, freelance magazine contributor, and has authored five automotive books. His background also includes racecar fabrication, muscle car and street rod restoration and construction, and operating his own automotive parts and repair business.

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