The difference between 5W-30 and 20W-50 motor oil is in their viscosity rating at different temperatures.The first number is a temperature reading, the second is the viscosity rating. Viscosity refers to how easily an oil pours at a given temperature, according to “How To Pick The Right Motor Oil For Your Car” by Paul Weissler.
An oil rated 5W-30 is a multi-viscosity oil that can be used over a range of temperatures. The W stands for winter, and 5 is for 5 degrees Celsius, the lowest temperature at which the oil will pour. The number 30 means the oil will have a viscosity rating of 30 at 100 degrees Celsius.