Multigrade engine oils are graded based on their viscosity compared to single grade oils. Grades, which are written as two numbers separated by the letter "W," show how well the oil performs in both cold and hot conditions. Some common grades are 0W-30, 0W-40, 5W-30 and 15W-40. Most commercial oils have a low number between 0 and 15 and a high number between 30 and 50.
The numbers in an oil grade are calculated using a complex formula that can be found on Carbibles.com. Checking the listed heat ranges for a single grade oil with the same number provides the ideal operating range of an oil. For instance, a grade of 0W-30 means that the oil performs as well as a 0-grade oil when cold and only thins as much as a 30-grade oil when hot. Carbibles.com also has a general guide to choosing oil based on the environmental temperature. In low temperatures, 0W-20 grade oil performs the best, with a range of -20 to 50 degrees Fahrenheit. In high temperatures, 15W50 is the best, with a range of -20 to 120 degrees Fahrenheit.
Oils are also grouped based on their chemical makeup, rating and category. Grade is only one of several things to consider when choosing oil.