Almost all industrial lubricants now come from base oils.
The American Petroleum Institute (API) has 5 base oil designations. The first three groups are derived from crude oil (mineral oil); Group IV base oils are fully synthetic; Group V is for all other base oils not included in Groups I through IV.
These base oils are less than 90% saturates, more than 0.03% sulfur and have an SAE viscosity index range of 80 to 120. Operating temperature range is from 32 to 150 F. These oils are solvent-refined, which is a simpler refining process, making these the cheapest base oils on the market.
Group II base oils are defined as containing more than 90% saturates, less than 0.03% sulfur, and have a V.I. of 80 to 120. These base oils are often manufactured by hydrocracking, which is a more complex process than solvent-refining. These oils have better antioxidation properties and have a clearer color than Group I base oils.
These base oils are greater than 90% saturates, less than 0.03% sulfur, and have a viscosity index above 120. Group III base oils are more refined than Group II and are typically severely hydrocracked (greater pressure and heat); this process produces a purer base oil.
These base oils are called polyalphaolefins (PAOs). They are synthetic and made through a process called synthesizing. PAOs have a broader temperature range and are great for use in applications exposed to extreme cold and/or high heat.
All other base oils that do not fall in the other groups are classified as Group V. Examples include silicone, phosphate ester, polyalkylene glycol (PAG), polyolester, and biolubes. These base oils can be mixed with other base stocks to enhance the oil’s properties.
These oils can be further classified into these categories, depending on composition: paraffinic, naphthenic, aromatic.