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.
A 20W-50 oil is a heavier multi-viscosity oil appropriate for higher temperatures and higher loads, which require a more viscous oil. The minimum temperature this oil will pour at is 20 degrees Celsius, and its viscosity rating at 100 degrees Celsius is 50.
Use in Cars
A 5W-30 oil is commonly used in passenger cars for normal use throughout the year. A 20W-50 oil will not protect a car as well in a cold winter and its greater viscosity will increase drag and friction in a passenger vehicle.