Motor oil grades are based on a rating that classifies oil by viscosity, with oils that have a higher viscosity creating a better seal and lubricant for moving parts, but also putting more stress on the engine since parts must work harder to move thicker oil. Oils rate for cold and hot temperatures, as well as performance level of the engine.
Oil grades vary by composition, with full synthetic oils having a higher viscosity designed for high performance vehicles. Premium conventional oils offer a range of temperature ratings for viscosity varying based on the demands of weather.
Created by the Society of Automotive Engineers, the oil rating appears on oil bottles in a series of numbers and letters that matches XW-XX for multi-grade viscosity oils or SAE X for non-winter ratings. The grade might read 10W-30 or SAE 5.
In the case of XW-XX ratings, the lower the first number, the less the oil thins in the cold, while the higher the second number, the thicker the oil. The W stands for winter, with lower numbers preceding the W marking oil better suited for colder climates. Manufacturers might add viscosity modifiers to thicken the oil.
A car's owner's manual recommends the right range of viscosity for the particular vehicle.