Base oils and process oils are the raw stock fluids, usually a refined petroleum fraction or a selected synthetic material, blended with additives to produce finished lubricants, greases, thermal oils, hydraulic fluid, and metal working fluids.

There are five specific categories, or groups, of base oils and process oils. These groups define the type of base stock from which the oil is formulated. Group I, or solvent freezing, base oils and process oils are the least refined of all the groups. Group I base oils comprise a mix of hydrocarbon chains and are typically used in low demand applications that don’t require a lot of stability and uniformity.

Group II and Group III are both known as hydro-processing and refining base oils and process oils. Group III base stocks formulate economic, mineral-based, semi-synthetic lubricants and have become more popular in the last ten years. Group III base oils are the most refined of the mineral based groups and are relatively stable and uniform.

Group IV, or chemical reaction base oils and process oils, are chemically engineered base stocks such as polyalphaolefins. Group IV base oils are highly stable and uniform when combined with certain additives and are commonly used in automotive and industrial applications. Group V oils are not used as base oils and process oils; rather, they are esters and polyesters that are used to make oil additives that contribute specific, beneficial characteristics to the finished lubricant. Oil additives improve the base oil’s viscosity, resistance to oxidation and corrosion, and can prevent foaming. Additives can also affect the oil’s pour point, or its ability to flow freely under low temperatures, and its flash point, or flammability.

Base oils and process oils in Groups I, II, II+ and III are widely used in combination with additives, esters, and polyalphaolefins to formulate semi-synthetic based lubricants. Synthetic base oils and process oils are designed with a molecular structure that has predictable properties, unlike mineral base oils and process oils, which are a mixture of naturally occurring hydrocarbons. Synthetic base oils and process oils have several advantages over mineral oils, including chemical stability, improved fuel economy in specific engines, better lubrication in low temperatures, and resistance to oxidation. However, synthetic base oils and process oils can decompose in certain industrial environments can crack plastic components that come in contact with them, and can come out of suspension in the leaded fuels still used in aviation applications.

Advances in processing technology over the years have made it possible to greatly improve the quality of base oils and process oils. Early processing simply removed impurities and aromatics; now processors can recover other components such as wax to make very high quality base oils and process oils.