The two oil types differ in their high temperature viscosity, and 10w40 is thicker than 10w30 when it comes to high temperatures. Before multi-grade oils were formulated, car owners typically used 10w30 for its wintertime flow properties, while relying on the 10w40 for the summertime when temperatures warmed up.
With oil, the letter "W" signifies "Winter" and provides an indication of how effectively the oil works during the winter months. The 10w30 oil consists of two viscocities, resulting in two grades of oil. Using oil with too high of a viscosity can result in an oil temperature that is too high, along with an increased amount of drag.