| Base Oil Category | Sulfur (%) | Saturates (%) | Viscosity Index | |
| Group I | >0.03 | and/or | <90 | 80 to 120 |
| Group II | <0.03 | and | >90 | 80 to 120 |
| Group III | <0.03 | and | >90 | >120 |
| Group IV | All polyalphaolefins (PAOs) | . | . | . |
| Group V | All others not included in Groups I, II, II or IV (Esters) | . | ||
| Base Oil Category | Manufacturing Method | Oil Characteristics |
| Group I | Solvent Freezing | Group I base oils are the least refined of all of the groups. They are usually a mix of different hydrocarbon chains with little or no uniformity. Due to their instability and poor overall properties, most of today’s engine oils use very little, if any, Group I base oils. They are generally used in less demanding applications. |
| Group II | Hydroprocessing and Refining | Group II base oils are common in mineral-based motor oils currently available on the market. They have improved lubricating performance over Group I base oils in areas such as volatility and oxidative stability. They have fair to poor performance in areas such as pour point and cold crank viscosity. |
| Group III | Severe Hydroprocessing and Catalytic Dewaxing | Group III base oils are subjected to the highest refining levels and while they originate from crude oil, they are chemically rearranged to offer molecular uniformity and stability for very good performance in a wide range of lubricating properties. They are commonly marketed as synthetic or mixed with lower group base oils and marketed as semi-synthetic products. Group III base oil products are increasing in popularity. |
| Group IV | Chemical Reactions | Group IV Polyalphaolefins (PAOs) are chemically engineered synthetic base stocks. When combined with additives, they offer excellent performance over a wide range of lubricating properties. They have very stable chemical compositions and highly uniform molecular chains. Group IV base oils are commonly found in automotive and industrial applications. |
| Group V | As Indicated | Group V base oils include all remaining non-synthetic and synthetic oils such as white oils, esters, silicones, and polyalkylene glycols. Esters are common Group V base oils used in lubricant formulations. Esters exhibit a wide variety of properties specific to each individual product. They are often used like additives to enhance the properties of other base oils. |
