10-30 vs. 5-40 motor oil

Updated April 17, 2017

Understanding motor oil nomenclature stamped onto every motor oil bottle could be the difference between keeping your vehicle's engine components running smoothly and increasing the likelihood of lubrication problems that lead to future repairs. The Society of Automotive Engineers (SAE), which classifies the viscosity of motor oil, helps the everyday driver understand how motor oils react in average and cold temperatures, which leads to a more informed motor oil buying decision.


Viscosity is the rate at which a substance flows. Highly viscous motor oil will flow slower than motor oil rated with a low viscous rating. The consistency of oil thins as it heats up and then thickens when cool. Motor oil rated at 5-40 has a higher viscosity at the average engine operating temperature compared to 10-30 and therefore thins less quickly at normal temperatures. According to Popular Mechanics, reasonably thick oil is a better sealant and creates a stronger film of lubrication between moving engine components. At the same time, oil must stay thin enough to flow smoothly between parts.

Motor Oil Testing

The Society of Automotive Engineers tests motor oil at -17.8 degrees Celsius and 98.9 degrees Celsius, which respectively correspond to the two ratings separated by a hyphen (10-30). The Society of Automotive Engineers decided that 98.9 degrees Celsius is the average engine operating temperature. The number without a "W" represents the SAE viscosity rating at 98.9 degrees C. For example, 10W-40 and 5W-40 motor oil react the same way during average engine operating temperatures but differently in cold weather.

Winter Grades

The "W" after the first number in motor oil nomenclature represents the word "Winter." Winter is generally a cold season so motor oils also must operate effectively in cold weather. Oil thickens when cold but still must stay fluid enough to keep engine components moving efficiently. The number before the "W" represents how well oil stays fluid at cold temperatures. 5W-40 won't thicken as much as 10W 30 at -17.8 degrees Celsius.

5-40 Usage

5W-40 motor oil runs well in cold temperatures. Popular Mechanics recommends 5W oil for winter use. A "synthetic" 5W-40 can sometimes flow smoother than "conventional" 5W-40 motor oils because of increased additives. The "40" corresponds to the SAE viscosity chart that starts at 20 and increases in increments of 10 until 60. Low viscosity oil also increases wear on engine components because it creates a thinner barrier between engine parts compared to high viscosity oil.

10-30 Usage

10W-30 motor oil will have a thicker consistency in colder weather but stay thinner in average engine operating temperatures compared to 5W-40 motor oil. 10W-30 motor oil may be appropriate for a driver who does not face harsh winter conditions.

Expert Insight

Every vehicle manufacturer lists the oil grade they feel is most appropriate for said vehicle, located in the owner's manual. Refer to this rating if you are unclear about what oil to use. Overly thick motor oil will reduce miles per gallon because the crankshaft has to use more energy to spin, as stated by Popular Mechanics.

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About the Author

Sean Russell has been a freelance writer since 2007, with experience featuring retailer product on blogs, creating press releases, SEO and publishing to the writing style of hundreds of websites. He graduated from Arizona State University with a Bachelor of Arts degree in creative writing and business, and achieved a California Real Estate license. He also enjoys motorcycling.