The different specifications of oil, or viscosity, refer to the resistance of the oil to flow, and the higher the number, the slower the oil flows. Motor oil comes in two main types, SAE or winter grades, which have the letter "W" on the bottle.
The W on a bottle of motor oil means that the oil viscosity went through testing at a colder temperature, and the numbers without a W go through testing at 210 degrees Fahrenheit. In other words, a motor oil with SAE 30 has the exact viscosity as an oil with 5W-30 or 10W-30, but only at 210 degrees F. In cold temperatures, a 5W-30 motor oil performs exactly like an SAE 5 oil, but still has the viscosity of the SAE 30.
Oil that flows more quickly gets to the engine faster, and the advantage of low winter viscosity is that the engine does not run dry, which means less wear. Some people do not prefer higher numbers when using oil, such as a 5W-30, because they believe that it is too thin, so they switch it with an SAE 30 or 10W-30. However, when in the engine, the viscosity is the same. The only time the 5W-30 is too thin is when it is cold, but that does not matter in an engine.