SAE 30 oil is a motor oil that has been given a viscosity rating of 30 by the Society of Automotive Engineers, according to AA1Car Automotive Diagnostic Help Center. Motor oils typically have ratings from 0 to 50.
Viscosity is a measure of how well an oil pours at a given temperature. Viscosity is sometimes referred to as weight. A low rating means a thinner oil, while a high rating means a thicker oil.
Thinner oils perform better in colder weather, especially when starting a car. Thicker oils perform better at higher temperatures. Single-grade oils, such as SAE 30, are thicker than lower grades, and in this case has a viscosity rating of 30 at a temperature of 100 degrees Celsius.
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SAE 30 oil is typically used for smaller air-cooled engines, like the ones in small tractors, lawnmowers, and chain saws. Most motor oils today are multi-grade oils that will perform well in all seasons.