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The American Petroleum Institute (API) has categorized base oils into five categories (API 1509, Appendix E). The first three groups are refined from petroleum crude oil.
Group I base oils are classified as less than 90 percent saturates, greater than 0.03 percent sulfur, and with a viscosity-index range of 80 to 120. The temperature range for these oils is from 32 to 150 degrees F. Group I base oils are solvent-refined, which is a simpler refining process.
Group II base oils are defined as being more than 90 percent saturates, less than 0.03 percent sulfur, and with a viscosity index of 80 to 120. They are often manufactured by hydrocracking, which is a more complex process. These oils have better antioxidation properties, a clearer color, and are more common today.
Group III base oils are greater than 90 percent saturates, less than 0.03 percent sulfur, and have a viscosity index above 120. These oils undergo more severe hydrocracking to achieve a purer base oil. Although made from crude oil, they are sometimes referred to as synthesized hydrocarbons.
