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Motor oils come in different weights or grades, and one of the most common for consumer vehicles is 10W-40. This designation refers to the viscosity of the oil in different temperature ranges.
Viscosity
Viscosity refers to how easily a given liquid flows. A liquid with low viscosity flows easily, while a high viscosity liquid resists flowing.
Viscosity Grades
The Society for Automotive Engineers (SAE) has developed a system for rating motor oil viscosity. The SAE viscosity scale runs from 5 to 50, with larger numbers indicating higher viscosity. For example, SAE 10 motor oil is runnier than SAE 20.
Effect of Temperature
Like most liquids, motor oil becomes less viscous when it heats up and becomes more viscous when it cools. As a result, oils with heavy weights, such as SAE 50, are used mostly in very hot climates. Low-viscosity grades are more common in cold climates.
Multigrade Oil
Motor oil grades that contain the letter "W" indicate multigrade oil. These products contain a low-viscosity motor oil as well as various polymer additives. The additives have little effect at low temperatures, but cause the oil to thin down in hot conditions more slowly than usual. The number before the "W" indicates the oils grade at 32 degrees Fahrenheit and the number after the "W" indicates the oils grade at 212 degrees Fahrenheit.
10W-40
The designation "10W-40" indicates a multigrade oil that is toward the low end of the SAE scale in cold weather, but at the upper end in hot conditions. 10W-40 oil is therefore good for a wide range of weather conditions.
