Weight and Oil Viscosity

Oil is like pancake syrup, on cool days oil would be as thick (viscous) as Mrs. Butterworth’s stored in the refrigerator. At high temperature, though, it would get uselessly thin, like microwaved syrup.

Long ago, the Society of Automotive Engineers (SAE) developed a rating system for oil viscosity, the most basic of characteristics, which is often called weight. It’s expressed by four or five characters, such as 10W-30. The lower the number before the W (for “winter”), the better it flows in cold temperatures. The number after the dash indicates how well the oil flows when it’s warm. Higher numbers mean the oil is “heavier,” or more viscous. Multi-viscosity oils developed several decades ago mean you don’t have to worry about using different oils for winter and summer.

Don’t select heavier-than-recommended oil in the mistaken belief it’ll provide better protection. Old-timers are used to heavier oils, but many manufacturers currently specify light oil, such as 5W-20. One reason is that some modern engines have incredibly tight clearances between parts. Especially when the engine is cold, a heavier oil may not reach into these tight areas. The lighter oil also helps provide better fuel economy.