Base Oil (Group I / II / III)
Ninety per cent of a finished lubricant is base oil, and which API group it comes from decides more about the product than the additive package does. The groups are defined by saturates, sulphur and viscosity index — three numbers with large consequences.
Molecular Structure
What is Base Oil (Group I / II / III)?
Base oil is the hydrocarbon fraction that makes up the bulk of a lubricant — typically eighty to ninety-nine per cent of the finished product, with the additive package accounting for the rest. It is a UVCB mixture of saturated hydrocarbons in roughly the C20 to C50 range, and its identity comes from the refining route rather than from a composition.
The American Petroleum Institute classifies base stocks into groups on three measurements. Group I is solvent refined, contains under ninety per cent saturates and over 0.03 per cent sulphur, with a viscosity index between 80 and 120. Group II is hydrocracked, reaching ninety per cent saturates or above and 0.03 per cent sulphur or below in the same viscosity index band. Group III is severely hydrocracked and hydroisomerised to a viscosity index of 120 or higher.
That progression matters because it tracks oxidative stability and volatility, which is what determines whether an oil can meet a modern engine oil specification and a long drain interval. Group I capacity has been closing steadily; Group II and Group III have grown, and Group III now competes directly with polyalphaolefin for fully synthetic positioning.
Highly refined base oils are exempt from carcinogen classification only if they pass IP 346 — a dimethyl sulphoxide extractables test that must give below three per cent. Unrefined and mildly refined mineral oils are classified as Category 1B carcinogens. That single test result determines the classification, the labelling and much of the handling regime, and it belongs on every certificate.
Quick Reference
Key Physical & Chemical Properties
Structure & Bonding
A mixture of saturated hydrocarbons — branched paraffins and alkyl-substituted naphthene rings — in the C20 to C50 range. The higher API groups contain progressively more iso-paraffin and less aromatic and sulphur-bearing material, and that shift is what raises oxidative stability and viscosity index.
Representative components — branched paraffin and alkyl naphthene
Structural Identity
- ProductLubricating oil base stock
- Substance typeUVCB — defined by refining route
- Carbon range≈ C₂₀ – C₅₀
- Group I processSolvent extraction and dewaxing
- Group II processHydrocracking
- Group III processSevere hydrocracking, hydroisomerisation
- Group IVPolyalphaolefin from 1-decene
- Key propertiesViscosity, viscosity index, pour point
- Carcinogen exemptionIP 346 DMSO extract below 3%
- CAS NumberVaries by group — verify per grade
- Specified byGroup, KV, VI, pour point, IP 346
Product Specifications
Chelora supplies Base Oil (Group I / II / III) in standard and custom grades. Contact us for specification sheets tailored to your process.
| Product | Base oil, API Group I, II or III |
|---|---|
| CAS Number | Varies with group and process — confirm with supplier |
| Common registry numbers | 64742-54-7; 64742-65-0; 64741-88-4 and others |
| API group | Declared per grade |
| Kinematic viscosity at 40 °C | Declared per grade, mm²/s |
| Kinematic viscosity at 100 °C | Declared per grade, mm²/s |
| Viscosity index | 80 – 120 for Groups I and II; ≥ 120 for Group III |
| Saturates | < 90% Group I; ≥ 90% Groups II and III |
| Total sulphur | > 0.03% Group I; ≤ 0.03% Groups II and III |
| Pour point | −9 to −18 °C typical, grade dependent |
| Flash point (COC, min) | 200 – 280 °C by grade |
| Colour (ASTM, max) | ≤ 1.0 for Group II and III |
| Noack volatility | Declared — governs engine oil consumption |
| Aniline point | Declared — indicates solvency and seal behaviour |
| IP 346 DMSO extract (max) | < 3.0 wt% — required for carcinogen exemption |
| Water content (max) | ≤ 0.05 wt% |
| Transport classification | Not regulated as dangerous goods |
Downstream Applications & Derivatives
Key derivative chains and industrial uses of Base Oil (Group I / II / III).
Engine Oils
Passenger car and heavy duty diesel engine oil, where Group II and III stocks are required to meet modern volatility, oxidation and long drain interval specifications.
Industrial & Hydraulic
Hydraulic fluid, turbine oil, compressor oil and circulating oil, where oxidative stability and consistent viscosity over life are the requirements.
Gear Oils & Greases
Industrial and automotive gear oils, and the base fluid in lithium and calcium sulphonate greases, where viscosity and additive solvency both matter.
Metalworking Fluids
Neat cutting oils and the oil phase of soluble and semi-synthetic emulsions, typically using Group I or II stocks selected for solvency.
Process & White Oils
Rubber and polymer process oils, and further refined white oils for pharmaceutical, cosmetic and food-grade applications under their own purity standards.
Transformer & Speciality
Insulating oil for transformers and switchgear, and speciality applications including heat transfer fluids and textile lubricants.
Why source Base Oil (Group I / II / III) through Chelora Petrochem?
Verified Origin
Full origin certification and asset-backed supply chain documentation, with API group, refining route and applicable CAS declared per grade.
Grade Flexibility
Group I, II and III stocks supplied across the viscosity range. We certify IP 346 on every lot, since that single figure determines classification and labelling for the whole downstream product.
Full Documentation
CoA including viscosity at both temperatures, viscosity index, pour point, saturates, sulphur, Noack and IP 346; SDS; REACH registration confirmation for the specific stream; and food-grade or white oil documentation where applicable.
Logistics Support
Experienced in bulk lubricant base stock logistics — ISO tanks, road tankers, flexitanks and drums, with heated handling where viscosity requires it, for Indian and international delivery.
Storage & Handling Guidelines
Always refer to the full SDS before handling. The following is a summary of key requirements.
- Confirm the API group and the applicable CAS number in writing. Base oil CAS numbers differ by refining route — solvent refined, hydrotreated, solvent dewaxed and hydrocracked streams each carry their own registry entry — and REACH registration and classification depend on the right one.
- Require IP 346 results on every lot. Below three per cent DMSO extract, the oil is exempt from carcinogen classification; above it, the material and everything formulated from it must be labelled as a Category 1B carcinogen.
- Do not substitute across groups without reformulating. Group II and III have lower aromatic content and therefore lower additive solvency and different seal swell behaviour — an additive package that dissolved cleanly in Group I may drop out of a Group III stock.
- Store in clean, dry tanks and control particulate ingress. Cleanliness at the base oil stage propagates directly into the finished lubricant, and filtration downstream is more expensive than exclusion upstream.
- Keep water out. Water causes haze, promotes additive hydrolysis and, in turbine and transformer applications, is a hard specification limit.
- Heat only as far as viscosity requires, and avoid prolonged high temperature with air contact, which oxidises the oil and darkens it before it ever reaches the blend.
- Segregate groups and viscosity grades rigorously. Base oils look identical, and cross-contamination is only discovered on test.
- Handle white and food-grade oils on dedicated equipment with documented cleaning, as their purity specifications cannot be recovered after contamination.
- Provide skin protection for routine handling. Highly refined oils have low toxicity, but prolonged contact with any mineral oil causes dermatitis and oil acne.
Hazard Summary
Classification depends on refining severity: highly refined base oils that pass IP 346 with under three per cent dimethyl sulphoxide extract are exempt from carcinogen classification and are generally not classified as hazardous. Unrefined and mildly refined mineral oils are classified as Carc. 1B. There is no middle ground, and the test result is the deciding evidence.
Identity is the documentation risk: base oil CAS numbers vary by refining route and by whether the stream is paraffinic or naphthenic. Solvent refined, hydrotreated, hydrocracked and solvent dewaxed streams each have their own entries. REACH registration, classification and customs declarations depend on the number matching the supplier's actual registration.
Group substitution is a formulation change: moving a blend from Group I to Group II or III changes additive solubility, elastomer seal swell and often the response to the same treat rate. The finished oil may fail a specification it previously passed. This is a reformulation exercise, not a like-for-like swap.
Dermatitis and oil acne: prolonged or repeated skin contact with any mineral oil, however well refined, causes dryness, dermatitis and folliculitis. Oil-soaked clothing held against the skin is a particular risk and should be changed rather than tolerated.
Mist and aerosol: oil mist generated by high-pressure spray, machining or compressor operation is a respiratory irritant with its own exposure limits, and requires extraction rather than reliance on general ventilation.
Aspiration: low viscosity grades present an aspiration hazard if swallowed. Do not induce vomiting.
Fire: a combustible liquid with a flash point between about 200 and 280 °C. Not a flammable liquid and not regulated for transport, but oil-soaked lagging and rags present a spontaneous heating risk, and heated systems should be assessed.
PPE minimum: safety glasses and oil-resistant gloves for routine handling, with extraction where mist is generated and prompt changing of contaminated clothing.
Consult the safety data sheet for the specific stream supplied and confirm IP 346 results before treating the material as unclassified.
Frequently Asked Questions
Technical and commercial questions about Base Oil (Group I / II / III) sourcing and specifications.
What are base oils used for?
They are the bulk of every lubricant. Engine oils for passenger cars and heavy duty diesel are the largest outlet, followed by industrial and hydraulic fluids, turbine and compressor oils, gear oils and the base fluid in greases. Beyond lubricants, base oils go into metalworking fluids, rubber and polymer process oils, transformer insulating oil, and — after further refining — white oils for pharmaceutical, cosmetic and food-grade use.
What do API Groups I, II and III actually mean?
They are defined by three measurements: saturates content, sulphur content and viscosity index. Group I has under ninety per cent saturates and over 0.03 per cent sulphur, with a viscosity index between 80 and 120, and is made by solvent extraction and solvent dewaxing. Group II reaches ninety per cent saturates or more and 0.03 per cent sulphur or less in the same viscosity index band, and is made by hydrocracking. Group III meets the Group II purity criteria with a viscosity index of 120 or above, achieved by severe hydrocracking and hydroisomerisation. Group IV is polyalphaolefin, a true synthetic made from 1-decene, and Group V is everything else including esters and naphthenics. The progression from I to III tracks oxidative stability and low volatility, which is what modern engine oil specifications demand.
Why does the CAS number vary?
Because base oils are UVCB substances identified by their refining process rather than by composition. A solvent refined heavy paraffinic distillate, a hydrotreated heavy paraffinic distillate, a solvent dewaxed heavy paraffinic distillate and a hydrocracked stream are four different registered substances with four different CAS numbers, even where the finished viscosity grades are similar. Naphthenic streams have their own entries again. Since the classification, the REACH registration and the customs declaration all follow the registry number, it has to match what the supplier actually registered for the stream being shipped — which is why the group and process route should be stated before a CAS is assigned to a product listing.
What is IP 346 and why does it matter so much?
IP 346 measures the proportion of the oil extractable into dimethyl sulphoxide, which correlates with polycyclic aromatic content — the fraction responsible for the carcinogenicity of poorly refined mineral oils. Under the applicable CLP note, a mineral oil is exempt from carcinogen classification if it can be shown that the DMSO extract is below three per cent. Above that, the material is Carc. 1B and everything formulated from it inherits labelling and handling consequences. So a single test result determines whether a base stock and the lubricants made from it are unclassified consumer-acceptable products or classified carcinogens. It belongs on every certificate of analysis without needing to be asked for.
How does Group III compare with polyalphaolefin?
Group III is a severely hydroprocessed mineral oil; PAO is a true synthetic built by oligomerising 1-decene. PAO has a narrower molecular weight distribution, better low-temperature fluidity, lower volatility at a given viscosity and superior oxidative stability, and it costs considerably more. Modern Group III has closed much of the performance gap and now meets many specifications that once required PAO, which is why a great deal of oil marketed as fully synthetic is Group III based. The remaining genuine advantages of PAO show up at temperature extremes and in very long drain or sealed-for-life applications. PAO also has weaker additive solvency than Group III, so ester co-base stock is common in PAO formulations.
What documentation does Chelora provide?
Every shipment includes a Certificate of Analysis covering API group, kinematic viscosity at 40 and 100 °C, viscosity index, saturates, total sulphur, pour point, flash point, colour, Noack volatility, aniline point, water content and IP 346 DMSO extract; a Safety Data Sheet for the specific refining stream; written confirmation of the CAS number and REACH registration applicable to that stream; food-grade or white oil documentation where relevant; an origin certificate; and destination-market certificates where required.
Request a quote or specification sheet
Talk to Chelora's sourcing team about Base Oil (Group I / II / III) grade, volume, logistics, documentation, and lead times. We respond within one business day.
