This article tells the main differences between 5-20 vs 5-30 vs 10-30 motor oil. Many of the new cars require a lower viscosity oil, for example 0-20 or 5-20. Also different oil viscosities are required for climates.
This is an excellent article that explains in detail about five (5) common myths that abound and are not true. For example, one that says to make your vehicle last longer, one should change their oil every 3 to 5 thousand miles… NOT TRUE!