The American Petroleum Institute (API) has divided base oils into five categories. The first two groups are refined from petroleum crude oil. They are mineral oils. Group IV base oils are full synthetic (polyalphaolefin) oils. Group V is ester.
Group I base oils are less than 90% saturates, greater than 0.03 percent sulfur and with a viscosity-index around 90. The temperature range for these oils is from 32 to 150 degrees F. Group I base oils are solvent-refined, which is a relatively easier refining process. This is why they are the cheapest base oils on the market.
Group II base oils are more than 90% saturates, less than 0.03 percent sulfur and with a viscosity index around 105. They are often manufactured by hydrocracking, which is a more complex process than Group I base oils. Because all the hydrocarbon molecules of these oils are saturated, Group II base oils have better anti-oxidation properties. They also have a clearer color and cost more in comparison to Group I base oils.
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 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 base oils are polyalphaolefins (PAOs). These synthetic base oils are made through a process called synthesizing. They have a much broader temperature range and are great for use in extreme cold conditions and high heat applications.
Group V base oils include 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. Ester oils can take more abuse at higher temperatures and will provide superior detergency compared to a PAO synthetic base oil, which in turn increases the hours of use.
Attention! Whichever base oil you choose, just ensure it appropriate for the application, temperature range and conditions in your vehicles.