If you work with lubricants — whether you're a blender, a buyer, or an engineer — you've seen base oils described as "Group I," "Group II," or "Group III." These labels come from a classification system created by the American Petroleum Institute (API), and understanding them is the first step to choosing the right base oil for any formulation.
In this guide, we'll break down all five API base oil groups, explain the technical differences that matter, and show you which group suits which application.
What is base oil?
Base oil is the main component of any lubricant, typically making up 70–99% of the finished product. The remaining fraction consists of additives — detergents, dispersants, anti-wear agents, viscosity modifiers and antioxidants — that give the lubricant its specific performance characteristics.
Base oils come from two sources: mineral base oils, refined from crude oil, and synthetic base oils, manufactured through chemical synthesis. The quality and type of base oil directly determine how the finished lubricant performs in terms of oxidation stability, low-temperature flow, volatility and service life.
The API base oil classification system
To bring order to a wide variety of base oils, the American Petroleum Institute (API) created a classification system in its publication API 1509. This system divides base oils into five groups based on three key parameters:
- Saturates content — the percentage of saturated hydrocarbons (measured by ASTM D2007). Higher saturates mean better oxidation stability.
- Sulphur content — the percentage of sulphur (ASTM D2622 / D4294). Lower sulphur means cleaner, more stable oil.
- Viscosity Index (VI) — how much the oil's viscosity changes with temperature (ASTM D2270). Higher VI means more stable viscosity across temperatures.
Groups I, II and III are all mineral base oils refined from crude oil — they differ only in refining severity and resulting purity. Group IV (PAO) and Group V (esters and others) are chemically distinct synthetic and specialty base oils.
The five base oil groups explained
Solvent-refined mineral base oil
The oldest and least processed mineral base oil, produced by solvent refining. Group I has the lowest purity of the mineral groups but remains widely used in industrial lubricants, marine oils and applications where extreme performance isn't required. It is also the most cost-effective.
Hydrocracked mineral base oil
Produced by hydrocracking, which removes more impurities than solvent refining. Group II has over 90% saturates and very low sulphur, giving better oxidation stability and a cleaner, water-white appearance. It's the modern workhorse for conventional and semi-synthetic engine oils.
Severely hydrocracked & hydroisomerized
The highest-purity mineral base oil, produced by severe hydrocracking and hydroisomerization. Group III has a viscosity index above 120 and performance approaching synthetic PAO — which is why it's often marketed as "synthetic" in finished oils. It's the preferred base stock for modern premium engine oils (API SP, ILSAC GF-6, ACEA C3).
Polyalphaolefin (PAO) — synthetic
The first true synthetic base oil group, polyalphaolefin is manufactured by chemically synthesizing ethylene-derived alpha-olefins. PAO offers excellent oxidation stability, very low pour point and high VI — making it ideal for full-synthetic engine oils, industrial gear oils and high-performance greases.
Esters, naphthenic & all other base oils
A catch-all category for every base oil that doesn't fit Groups I–IV. This includes ester base oils (polyolesters, diesters), naphthenic base oils, polyalkylene glycols (PAG), silicones and more. Group V oils are often blended with PAO to enhance additive solubility, or used alone for specialty applications.
Side-by-side comparison table
Here's how the five groups compare across the key parameters that define them:
| Group | Type | Saturates | Sulphur | VI | Typical Use |
|---|---|---|---|---|---|
| Group I | Mineral (solvent-refined) | <90% | >0.03% | 80–120 | Industrial, marine, process oils |
| Group II | Mineral (hydrocracked) | >90% | <0.03% | 80–120 | Conventional & semi-synthetic engine oils |
| Group III | Mineral (VHVI) | >90% | <0.03% | >120 | Premium synthetic-grade engine oils |
| Group IV | Synthetic (PAO) | 100% | ~0 | 130–145 | Full-synthetic, racing, gear oils |
| Group V | Synthetic / specialty | Varies | Varies | Varies | Esters, transformer, refrigeration, RPO |
The numbers tell a story: as you move from Group I to Group III, saturates rise and sulphur falls — the oil gets purer and more stable. Group IV and V are an entirely different chemistry, made (not refined) for performance that mineral oils can't reach.
How to choose the right base oil group
Selecting a base oil group comes down to matching performance requirements against cost. Here's a practical guide:
- Choose Group I when cost matters most and the application is undemanding — industrial lubricants, marine engine oils, rubber process oils, and as a feedstock for further processing.
- Choose Group II for conventional and semi-synthetic automotive oils, hydraulic fluids and premium industrial lubricants needing good oxidation stability at moderate cost.
- Choose Group III for modern premium engine oils requiring high VI and low volatility — meeting API SP, ILSAC GF-6, ACEA C3, dexos1 and similar specifications, without the cost of full synthetic.
- Choose Group IV (PAO) when you need extreme low-temperature performance (below −40°C), very high thermal stability, or full-synthetic OEM positioning.
- Choose Group V (esters, naphthenic) for specialty applications: transformer oil, rubber process oil, refrigeration oil, metal-working fluids, or as a co-base with PAO.
Need help selecting the right base oil?
Sinolook supplies all five API base oil groups — from Group I solvent-refined oils to ADNOC ADbase Group III and naphthenic specialty oils. Our technical team can recommend the right grade for your formulation.
Talk to Our Technical TeamFrequently asked questions
How many groups of base oil are there?
The American Petroleum Institute (API) classifies base oils into five groups. Groups I, II and III are mineral base oils refined from crude oil, distinguished by saturates content, sulphur level and viscosity index. Group IV is polyalphaolefin (PAO), a synthetic hydrocarbon. Group V is a catch-all category for all other base oils — including esters, naphthenic oils, polyalkylene glycols and silicones.
What is the difference between Group I, II and III base oil?
The key difference is refining severity and resulting purity. Group I is solvent-refined (saturates <90%, sulphur >0.03%, VI 80–120). Group II is hydrocracked (saturates >90%, sulphur <0.03%, VI 80–120). Group III is severely hydrocracked and hydroisomerized (saturates >90%, sulphur <0.03%, VI >120). As processing severity increases from I to III, you get better oxidation stability, lower volatility and higher purity. Read our full Group I vs II vs III comparison →
Which base oil group is best for engine oil?
For modern passenger car engine oils meeting API SP, ILSAC GF-6 or ACEA C3, Group III base oil such as ADNOC ADbase is usually preferred — offering high VI and low volatility at reasonable cost. Group II is widely used for conventional and semi-synthetic oils. Group IV PAO and Group V esters are used in premium full-synthetic and racing oils. The best group depends on the target specification, budget and operating conditions.
Is Group III base oil synthetic?
Technically, Group III is a mineral base oil because it's refined from crude oil, not chemically synthesized. However, due to its very high purity and performance approaching PAO, Group III is often marketed as "synthetic" or "semi-synthetic" in finished lubricants — and in some markets this labeling is legally permitted. True synthetic base oils are Group IV (PAO) and Group V (esters).
What group is naphthenic base oil?
Naphthenic base oil falls under API Group V, the catch-all category. Naphthenic oils are mineral base oils rich in cyclic saturated hydrocarbons, with very low pour point and excellent solvency. They're used in transformer oil, rubber process oil, refrigeration oil and metal-working fluids rather than engine oils.
Understanding the five base oil groups is the foundation for every lubricant decision. Whether you need cost-effective Group I, versatile Group II, premium Group III, or specialty Group V oils, choosing the right group ensures your finished lubricant performs as intended.