All lubricants consist of base oil. Normally, lubricants consist of 90% base oil and 10% additives. The American Petroleum Institute (API) divided base oils into five groups determined by saturation level, sulfur level and viscosity index. We will examine the first three groups
Group I oils are solvent-refined, which is a simpler refining process, making them the least refined and therefore also the cheapest base oils available. Solvent-refined oils consist commonly of a mix of different hydrocarbon molecules which cannot be distinguished in the refining process. This results in an oil with irregular molecules causing increased friction within the oil. Group I oils are therefore used most often in less demanding applications.
Group II base oils undergo hydrocracking which is a more complicated process than the process for Group I oils. Hydrocracking is a process that breaks down large hydrocarbon molecules into smaller ones. The hydrocarbon molecules of these oils are saturated, giving them better antioxidation properties. Group II oils are priced closely to Group I oils.
Group III oils undergo an even longer process than Group II oils. The process, called severe hydrocracking, is also more intense. More pressure and heat is applied during the refinery process. This results in a purer base oil with a higher quality. Even though Group III oils are derived from crude oil, they are sometimes described as synthesized hydrocarbons.
