Base Oils and API Groups
Base oil makes up 60% to 98% of a finished lubricant. API's five base oil groups, the criteria, production routes and the move to Group II in marine oils.
Base oil is the refined oil that forms the main body of a lubricant or grease, typically 60% to 98% of the formulation.[6] A finished product is made by blending base oil with an additive package, and in some engine oils the additive share can exceed 20%. The American Petroleum Institute sorts base oils into five groups in Appendix E of its Publication 1509, on the basis of saturates, sulphur and viscosity index.[1] Marine cylinder oils were built on Group I for a long time. As Group I capacity shrinks, formulations are moving towards Group II.
What a Finished Lubricant Contains
A finished oil is base oil plus additives. The base oil forms the lubricating film, carries heat and keeps additives in solution. Additives bring what it cannot do alone. Detergents keep metal surfaces clean and supply the ability to neutralise acid, dispersants hold soot in suspension and antioxidants slow ageing. Anti-wear agents protect metal, viscosity modifiers improve temperature behaviour and pour point depressants help cold flow. Grease rests on the same base, with a thickener holding the oil in place.
The API 1509 Classification
| Group | Saturates (%) | Sulphur (%) | Viscosity index | Main production route |
|---|---|---|---|---|
| I | Below 90 and/or | Above 0.03 | 80 to 120 | Solvent refining |
| II | 90 or more | 0.03 or less | 80 to 120 | Hydrocracking |
| III | 90 or more | 0.03 or less | 120 or more | Severe hydrocracking, hydroisomerisation |
| IV | All polyalphaolefins (PAO) | Chemical synthesis | ||
| V | All base oils not in the other groups | Naphthenics, esters, PAG and others | ||
Source: API 1509 Appendix E values as reproduced by Lubes'n'Greases.[1][2] The "and/or" in the Group I line matters, as an oil meeting either condition falls into that group.
The Three Criteria
Saturates are the share of molecules with no double bonds or aromatic rings. These molecules resist reaction with oxygen, so a higher share means better oxidation stability. An oil that oxidises thickens, turns acidic and leaves deposits.
Sulphur is given as a percentage, with 0.03% equal to 300 ppm. Sulphur compounds can act as natural antioxidants, but they also mark impurities left from the crude. Hydrogen treatment removes most of them.
Viscosity index (VI) is a dimensionless number for how much an oil's viscosity changes with temperature. It is calculated from viscosity at 40 °C and 100 °C and compared against two reference oils.[2] A high VI oil thins less when hot and thickens less when cold. The groups are not a ladder of quality, since the right choice depends on the application and the additive package.
Standard test methods apply: ASTM D2007 for saturates, ASTM D1552, D2622, D3120 or D4294 for sulphur, and ASTM D2270 for viscosity index. Viscosity grade is separate information. Group I neutrals are traded as grades such as SN 150 and SN 500, with bright stock at the heavy end of that range.
How Base Oil Is Made
After atmospheric distillation, crude goes to vacuum distillation. Lighter cuts go to fuels, and the heavy vacuum cuts become the feed for base oil. Paraffinic crude is the most common feedstock and gives a high VI. Naphthenic crude yields products that flow better at low temperature. The wider process is described in Oil Refinery.
Group I: Solvent Refining
A solvent extracts unwanted aromatic compounds from the oil, and dewaxing then corrects its flow properties. The molecules themselves are largely unchanged, so some sulphur, nitrogen and ring compounds remain. Colour runs from amber to deep gold and VI typically falls between 90 and 105. The heaviest cut, bright stock, is sought after in marine and heavy industrial oils for its high viscosity and has no direct equivalent in the other groups.[3]
Group II: Hydrocracking
Under high pressure and temperature, hydrogen removes sulphur and nitrogen, saturates aromatic rings and breaks up large molecules. The result is a clear, colourless oil. Oxidation stability is better than Group I, but solvency, meaning the ability to keep additives in solution, is lower. That is the main reason the change calls for reformulation. Group II comes from petroleum and counts as a mineral oil.
Group III: Severe Hydrocracking
The process runs at higher severity and finishes with hydroisomerisation, which converts straight-chain molecules into branched ones and lifts VI above 120. Since it starts from crude it is technically mineral, even though some markets sell it as synthetic. The technical data sheet shows which group a label really means.
Groups IV and V
PAOs are synthesised from olefins in chemical plants. A high VI and very low pour point suit them to extreme temperatures, and they cost well above Group III. Group V gathers everything else: naphthenics, silicones, phosphate esters, polyalkylene glycols and polyol esters. Esters are often added to improve another base oil.
Names such as Group II+, Group III+ and Group VI appear in the market but not in the API table. They are marketing terms.[2]
Base Oil in Marine Lubricants
In a two-stroke marine engine, cylinder oil is injected onto the liner and consumed in combustion, so volumes are large. An engine running at 20,000 kW with a feed rate of 0.7 g/kWh uses 14 kilograms of cylinder oil an hour, or about 336 kilograms a day. The rate varies by engine and oil, but the sum shows why cost matters.
One of the core jobs of cylinder oil is neutralising acid formed from sulphur in the fuel, a capacity measured as base number (BN). The unit is mg KOH/g, so BN 40 means an alkaline reserve equal to 40 milligrams of potassium hydroxide per gram of oil. Base oil has no BN of its own. Detergents in the additive package supply it, and the base oil keeps them in solution. When IMO 2020 cut fuel sulphur, less acid formed and fleets moved from BN 70 and BN 100 oils towards BN 40. MAN ES, now Everllence, divided cylinder oils into Category I and Category II, and specified Category II for Mk 9 and later engines.[4]
Supply tightened in the same period. Infineum, citing ICIS, reports the loss of 2,395 thousand tonnes of Group I capacity to refinery closures since 2020.[4] Since 2014, nine plants in Europe, the Middle East and Africa have partly or fully left Group I.[3] Lubricant makers are responding with additive packages designed for Group II. Infineum launched its 40 BN Category II package for Group I in 2022 and went on to test it with Group II.[4] Replacing bright stock is harder, as heavy viscosity and solvency are lost together, so formulations are sometimes supported by high viscosity naphthenic oil. Product ranges for ships are on the Marine Oils page.
Market Conditions in 2026
The base oil market felt the Hormuz crisis in 2026. Argus reported on 9 October that Middle Eastern flows, which account for as much as 60% of European imports, were seriously disrupted by the effective closure of the strait. Group III prices climbed through the summer and into September, reaching record highs. Groups I and II fell in the third quarter, as buyers had stocked up at high prices earlier in the year.[5] Such moves follow the stock cycle and can reverse within months, while the loss of Group I capacity is a long-term trend that is permanently changing marine oil formulations.
Frequently asked questions
What is base oil?
Base oil is the refined oil that forms the main body of a lubricant or grease, typically 60% to 98% of the formulation. Additives make up the rest.
What are the base oil groups?
API defines five. Group I is solvent refined, Group II hydrocracked and Group III severely hydrocracked. Group IV is PAO, and Group V gathers all the others. The limits are set by saturates, sulphur and viscosity index.
What is base oil used for?
It forms the lubricating film, carries heat and keeps additives in solution. Engine, marine and industrial oils and greases are all built on it.
What is the difference between base oil and lube oil?
Base oil is the raw material. Lube oil is the finished product, made by blending base oil with an additive package.
Is Group III a synthetic oil?
The API table has no synthetic group. Group III comes from petroleum and is technically mineral, though some markets label it synthetic. The chemically synthesised PAO is Group IV.
Can Group II replace Group I?
Not without reformulation. Solvency and heavy viscosity differ, so a new additive package or blend is needed.
What is bright stock?
A very high viscosity heavy base oil produced on Group I plants and used in marine and heavy industrial lubricants.
Sources
- Lubes'n'Greases. Base Oils Defined, Factbook (reproducing API 1509 Appendix E).
- Lubes'n'Greases. Base Oil Properties, Factbook; Machinery Lubrication. Base Oil Groups Explained.
- Lubes'n'Greases. Reaching the End of the Group I Barrel; Bright Stock in Short Supply; Time to Rethink Bright Stock.
- Infineum Insight. Built-in Flexibility and Performance; Nynas. Time to Reformulate as Group I Departs; gCaptain. Chevron Marine Lubricants Testing of New Marine Cylinder Oil.
- Argus Media. European Base Oils: Group III Price Pressure, October 2026.
- Lubes'n'Greases. Introduction to Lubricants, Factbook.
