Motor oil viscosity (defined as a fluid’s resistance to flow) is expressed in a numerical grading system set by the Society of Automotive Engineers (SAE) and labeled as their SAE code. Examples of the codes for single weight oils are 10W, 30, 40 or 50.

Multi grade oils might carry labels like 5W-30, 10W-40 or 15W-40. The “W” designation stands for “Winter”, not weight, and is determined by the oil’s flow characteristics at various cold temperatures.

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