PolymerTrade Guide
Group I through Group V explained
Base oil groups determine performance, cost and marketing claims for finished lubricants. Here is what actually separates one group from the next.
The American Petroleum Institute (API) classifies base oils into five groups according to three measurable properties: saturate content, sulphur content and viscosity index (VI), a measure of how much an oil's viscosity changes with temperature. Higher viscosity index means the oil holds its viscosity more consistently across a temperature range, which generally means better lubricant performance across operating conditions.
| Group | Process | Viscosity index | Typical use |
|---|---|---|---|
| Group I | Solvent refined | 80–120 | General industrial oils, lower-spec lubricants |
| Group II | Hydroprocessed | 80–120 | Automotive engine oils, hydraulic fluids |
| Group III | Severely hydrocracked | 120+ | Premium engine oils, marketed as synthetic |
| Group IV | Chemical synthesis (PAO) | 120+ | High-performance synthetic lubricants, extreme temperature range |
| Group V | All other base stocks (esters, naphthenics, PAGs) | varies | Process oils, specialty and niche applications, often blended with other groups |
Group I base oils are produced through conventional solvent refining rather than hydroprocessing, giving them the widest specification tolerance and lowest saturate content of the paraffinic groups. Group I supply has been shrinking globally as refiners shift toward Group II and III capacity, but Group I remains cost-effective for general industrial oils and lower-specification lubricant applications where the tighter performance of Group II or III is not required.
Group II base oils are hydroprocessed (treated with hydrogen under pressure to remove impurities and improve stability), giving better oxidation resistance and a cleaner, lower-sulphur oil than Group I at a similar viscosity index range. Group II has become the dominant base oil group globally for automotive engine oils and hydraulic fluids.
Group III base oils go through severe hydrocracking and hydroisomerization, pushing the viscosity index above 120, which approaches the performance of Group IV synthetic PAOs at a meaningfully lower production cost. This is why Group III oils, despite being derived from petroleum feedstock rather than built through discrete chemical synthesis, are permitted to be marketed as "synthetic" in finished lubricants in most major markets, including the United States.
Group IV covers polyalphaolefins (PAOs), true synthetic base stocks built through controlled oligomerization of alpha-olefins, giving the most consistent molecular structure and the widest useful temperature range of any group. Group V is a catch-all for every base stock that does not fit Groups I through IV, including esters, polyalkylene glycols (PAGs) and naphthenic oils.
Naphthenic oils, the most commonly traded Group V category on PolymerTrade, have naturally low pour points and strong solvency, making them well suited to process oils, transformer oils, rubber-processing oils and certain metalworking fluids, applications where paraffinic groups (I through IV) are not the natural fit.
Choosing a base oil group starts with the finished-product performance requirement (viscosity index, oxidation stability, temperature range) and the marketing claim needed (conventional versus synthetic), then works back to the group and grade that meets both at the lowest acceptable cost. A certificate of analysis per batch, which every base oil listing on PolymerTrade carries, confirms the actual measured properties against the group specification rather than relying on the group label alone.
Browse Group I to Group III base oils, naphthenics, process and white oils on PolymerTrade.