Oil grade (or weight) is probably the single most important factor in choosing an engine oil. The grade numbers on the bottle correspond to the oil's viscosity, or thickness. The smaller the number, the thinner the oil. Multigrade oils are popular because, under all but very extreme conditions, they are thin enough to flow well in cold weather but still retain enough thickness for high-temperature lubrication. In fact, the "W" designation on a multigrade bottle, such as 10W-30, stands for "winter," or the oil's flow characteristics in cold temperatures. A single-grade oil is recommended only for use in a narrow set of temperature conditions.