What Do the Numbers and Letters on Oil Stand For?

Initials & numbers on the labels of oil containers describe the viscosity & quality of the oil

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When choosing oil for your car, make sure that it has been approved by the American Petroleum Institute. This is indicated by a starburst on the label and the abbreviation API. Letters such as SJ, SL or SG refer to the service classification of the oil. API has deemed classifications SA through SH obsolete, so only purchase SJ through SN motor oil (unless you’re driving a Model T, in which case SA is perfect for you). For diesel-powered vehicles, the S is replaced by a C.

The numbers on your oil (e.g. 10W-40) describe its viscosity. The first number gives a designation for the oil in cold weather (the W stands for “Winter”) and the second number gives a designation for when the oil heats up. These two numbers vary depending on what you’re driving and what kind of environment you’re driving in.

Oil thickens as it cools and thins as it heats up. The higher that second number, the thicker your oil will remain at high temperatures. Thick oil is good because it provides better lubrication and better sealants for the engine. However, if the oil is too thick it will decrease your fuel economy by making your engine work harder to turn the crankshaft. The first number informs you how cold your oil can get before thickening to the point of inefficiency. To get the best performance out of your car, check your owner’s manual for the appropriate oil weight.