The measure of a liquid’s ability to flow under varying temperature conditions. In automobiles, viscosity most often refers to the “weight” of a motor oil, which is designated using number and letter grades established by the Society of Automotive Engineers. Oils with low numbers such as 5W or 10W flow easily at low temperatures (the “W” stands for winter). Oils with high numbers such as 30 or 40 resist thinning at high temperatures. Most modern engines call for multi-grade oils such as 5W-30 that perform well at both low and high temperatures.