The American Petroleum Institute (API) has classified base oils into 05 categories (API 1909, Appendix E). The three groups I, II, and III are are refined from

petroleum crude oil. Group IV base oils are full synthetic (Polyalphaolefin, PAO). Group V is for all other base oils not included in groups I, II, III, and IV.

Group I

Group I base oils have < 90% saturates, > 0.03% sulfur and viscosity index (VI) from 80 to 120. Group I base oils are solvent-refined, which is a simpler refining process. This is reason for its prices are the cheapest.

Group II

Group II base oils have > 90% saturates, < 0.03% sulfur and VI from 80 to 120. They are often produced by hydrocracking, which is a more complex process than what is used for group I. Due to all the hydrocarbon molecules of these oild are saturated, group II have better antioxidation properties. They also have a more clear color and higher cost in comparison to group I. Today, group II base oils are becoming very common on the market and are priced very close to group I.

Group III

Group III base oils have > 90 % saturates, < 0.03 % sulfur and have VI above 120. These oils are refined even more than Group II base oils and generally are severely hydrocracked (higher pressure and heat). This longer process is designed to achieve a purer base oil. Although made from crude oil, Group III base oils are sometimes described as synthesized hydrocarbons. Like Group II base oils, these oils are also becoming more prevalent.

Group IV

Group IV base oils are Polyalphaolefins (PAOs). These synthetic base oils are made through a process called synthesizing. They are tolerate both lower and higher temperatures, and great for use in extreme cold conditions and high heat applications.

Group V

Group V base oils are classified as all other base oils, including silicone, phosphate ester, polyalkylene glycol (PAG), polyolester, biolubes, etc. These base oils are at times mixed with other base stocks to enhance the oil’s properties. An example would be a PAO-based compressor oil that is mixed with a polyolester.

Esters are common Group V base oils used in different lubricant formulations to improve the properties of the existing base oil.

Please remember, you should select base oil appropriate to your application, temperature range and conditions.

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