What Does the "5W-40" in Motor Oil Mean?

by Jerry Romick

Motor oil provides lubrication to a vehicle's engine, keeping the moving parts from overheating due to friction. It also transfers heat from the combustion cycle, helps keep the engine clean by suspending combustion by-products and protects the engine from oxidation. Ambient air temperature and the internal temperature of a running engine affect how well a motor oil performs these functions. Understanding an oil's weight, or viscosity, is helpful in choosing an oil that is appropriate for the driving conditions you expect to encounter.

The American Petroleum Institute Certification Mark

A can of automotive motor oil will normally have two labels. The first is the American Petroleum Institute certification mark or "starburst." This label indicates that the oil has met current engine protection and fuel economy standards as established by the International Standardization and Approval Committee, which is a joint venture between U.S. and Japanese automakers.

The American Petroleum Institute Service Symbol

The API service symbol, also called the "donut," is the second label on a can of motor oil and is made up of three parts. The upper most portion of the service symbol identifies the API service category within which the oil falls: "S" for gasoline or "service" engines, and "C" stands for diesel or "commercial" engines; the "S" or "C" will will be followed by another letter, which the API updates on a regular basis to keep up to date with the changing needs of new automobile engines; the most recent category is "SN," which was introduced in October 2010 and identifies the motor oil as suitable for vehicles manufactured in 2011 or earlier. The middle section of the donut indicates the Society of Automotive Engineers viscosity, or weight rating, of the oil. The bottom third of the label identifies the oil as resource or energy conserving.

What the Numbers Indicate

The two numbers in the 5W-40 indicate the weight or viscosity of the engine oil as tested by SAE. Viscosity is defined as a fluid's resistance to flow: The higher the number, the higher the fluid's resistance to flow, or the thicker the fluid. Oils with two numbers, such as 5W-40, are multi-grade oils; they exhibit the characteristics of an oil with a viscosity rating of 5 at zero degrees Fahrenheit and the characteristics of an oil with a viscosity rating of 40 at 212 degrees Fahrenheit. The "W" indicates that the first number is the "cold" or "winter" rating.

How One Oil Can Have Two Viscosity Ratings

Oil responds to temperature by thickening when cold and thinning when hot. Single-grade oils, those with a single rating from SAE, will either be too thick to properly lubricate an engine when cold, or get too thin as the engine reaches operating temperature. Oil manufacturers introduce additives to a thin base oil that cause a 5W-40 oil to act like a single-grade oil with a five rating when cold, but then as the oil heats up it acts like a single-grade oil with a 40 rating.

About the Author

Jerry Romick has worked in radio and television for more than 30 years, often contributing to radio publications and websites. He is also an avid motorcyclist who has written about motorcycles for sites such as AllAboutBikes and PowerSportsTV. Romick holds a Bachelor of Science in communications from West Liberty State College.

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