The weight, or viscosity, of a motor oil determines the labeling it is sold under. Weights such as 5W-20 and 5W-30 are very close numerically, but have distinct viscosity differences
The numbering system of the motor oil industry normally gives two values within the label. For example, a 5W-20 rating signifies that its cold pour point is at 5 degrees Celsius. The W stands for the grade type, which is winter; and the 20 is the viscosity index expressed at 100 degrees Celsius.
The difference between 5W-20 and 5W-30 is that the lower viscosity in 5W-20 makes it a base stock oil that is very thin and geared toward winter use. In contrast, 5W-30 has more oil additives, as well as thicker base stock oil, making it a bit difficult to stay within its weight grade as a vehicle ages with mileage.
Overall, both 5W-20 and 5W-30 weight oils are used during all seasons of the year, mainly for overhead cam engines.
