Base Oil Group I, II and III Explained: A Buyer's Guide to API Categories

API base oil groups are defined by three numbers: the percentage of saturates, the sulphur content and the viscosity index (VI). Group I is solvent-refined and has the lowest saturates and highest sulphur; Group II is hydroprocessed, with at least 90% saturates and no more than 0.03% sulphur; Group III has the same limits as Group II but a viscosity index of 120 or higher. Viscosity grades such as SN150, SN500 and bright stock tell you how thick the oil is, and are a separate question from the group.

For a buyer, the practical consequence is that the group sets what the oil can be used for and how it will behave under heat and oxidation, while the viscosity grade decides which finished lubricant you can blend. This guide explains the API categories, the common grades, where each is used and which lines on a certificate of analysis (CoA) deserve your attention. For availability and specifications, see our base oil product page.

The short answer: how the groups differ

The American Petroleum Institute (API) classifies mineral base oils into Groups I, II and III by composition and viscosity index. Groups IV and V cover synthetic and other oils.

Group IV is polyalphaolefin (PAO), a fully synthetic base oil. Group V is everything not covered by Groups I to IV, including esters, alkylated naphthalenes and many other specialty fluids. Those two groups are outside the scope of this article.

Two points are worth stating at the start.

• The group is a floor, not a quality guarantee. It tells you the oil passes minimum thresholds. Two Group II oils from different refineries can differ in colour, volatility and oxidation stability.
• The group does not tell you the viscosity. A Group I oil and a Group II oil can both be a light or a heavy grade. You need the viscosity grade as well.

How each group is made, and why it matters

Group I: solvent refined

Group I oils are produced by distilling crude oil into lubricating fractions and then using solvents to remove aromatics and waxes. The result is a base oil with relatively high aromatics and sulphur. These compounds give Group I certain benefits, notably good solvency for additives and for the sludge that forms in service, but they also make the oil less resistant to oxidation.

Group I has been declining in global supply as refiners shift capacity to hydroprocessing, but it remains widely traded and is still the only route to certain heavy grades, notably bright stock.

Group II: hydroprocessed

Group II oils are made by hydroprocessing, in which hydrogen under pressure and a catalyst remove sulphur and nitrogen and saturate most of the aromatics. The result is a clearer, more oxidation-stable oil with low sulphur. Group II is the dominant base oil for passenger-car engine oils and many industrial lubricants in markets that have moved to modern specifications.

Group II oils usually have low solvency compared with Group I, which formulators manage with additive choice. They are generally lighter in the viscosity range, and heavy Group II grades are less common than heavy Group I.

Group III: severely hydrocracked or hydroisomerised

Group III oils go through more severe hydrocracking, or are made by hydroisomerising waxes, which rearranges the molecules to give a higher viscosity index and very high saturates. They are often marketed as high-performance mineral or "synthetic technology" base oils because their properties approach those of PAO in several respects, such as low volatility and good low-temperature behaviour, at a lower cost.

Group III is generally supplied in lower-viscosity grades, measured in centistokes at 100°C, rather than the SN grade system used for Group I.

Informal sub-groups

You will see "Group II+" and "Group III+" in offers. These are not official API categories. They are marketing terms for oils that exceed the minimum of the group, usually with a higher VI or better volatility. Treat them as a cue to ask for the actual figures, not as a specification in themselves.

The three numbers behind the groups

Saturates

Saturates are hydrocarbons without double bonds, mainly paraffins and naphthenes. They resist oxidation better than aromatics, which is why higher saturates generally means a longer-lived lubricant. The API threshold is 90% saturates.

Sulphur

Sulphur content is measured by weight. The threshold is 0.03% between Group I and Groups II/III. Sulphur compounds in Group I are not all harmful, since some contribute natural antioxidant behaviour, but low sulphur matters for catalysts in modern emission-control systems and for the performance expectations of modern engine-oil specifications.

Viscosity index

Viscosity index (VI) describes how much viscosity changes with temperature. A higher VI means the oil thins less as it heats up. The thresholds are 80 to below 120 for Groups I and II, and 120 or above for Group III. A high-VI base oil reduces the need for viscosity-index-improver polymers in the finished lubricant, which is one of the reasons Group III is favoured for low-viscosity, fuel-economy engine oils.

Viscosity grades: SN150, SN500 and bright stock

The grade system you will see most often in trade, especially for Group I, uses the prefix SN, standing for "solvent neutral". The number is a rough guide to viscosity, historically related to Saybolt Universal Seconds at 100°F. The exact figures differ by refinery, so always check the CoA and the specification rather than relying on the grade name.

• SN150. A light grade, used for engine oils, hydraulic fluids and other thin lubricants. Roughly 30 cSt at 40°C in typical Group I production.
• SN300 and SN500. Medium grades. SN500 is a common choice for industrial gear oils and heavier engine-oil formulations, and is widely used as a blending component. Typical kinematic viscosity at 40°C is on the order of 100 cSt for SN500 in many specifications.
• Bright stock. The heaviest Group I grade, made by deasphalting and refining the vacuum residue. It is characterised by viscosity at 100°C, typically in the region of 30 cSt or higher, rather than at 40°C. Bright stock is used to thicken blends, for example in gear oils, marine cylinder oils, greases and some industrial oils.

Group II suppliers tend to use grade names such as 150N and 500N, or quote viscosity at 100°C. Group III grades are normally named by 100°C viscosity (for example, a 4 or 6 cSt grade). The "N" naming is also not strictly standardised, so match grades by measured viscosity, not by name.

Decision rule: if a formulator quotes "SN500" for a blend, ask for the target kinematic viscosity at 40°C and 100°C, and verify that the offered cargo meets it. Different refineries sell SN500 with different viscosities and different VI, and the difference shows up in the finished lubricant.

Applications: matching group and grade to the lubricant

The groups are not simply "good, better, best". Each has a place.

Engine oils

• Passenger-car and light-duty engine oils in modern specifications are most commonly built on Group II and Group III base stocks, with Group III favoured for low-viscosity grades that need high VI and low volatility.
• Heavy-duty diesel engine oils use a mix of Group II and Group III, depending on the specification and the formulator's approach.
• Older or lower-specification engine oils, and engine oils in markets where Group I remains in widespread use, may be built on Group I blends.

Industrial oils

• Hydraulic oils are formulated on Group I or Group II. Group II is used where better oxidation stability and cleanliness are required.
• Gear and compressor oils use medium and heavy grades such as SN500 and bright stock in Group I, or Group II equivalents, depending on the performance level.
• Turbine and circulating oils usually require high oxidation stability and low sludge, which favours Group II.
• Process oils and metalworking fluids use a range of grades. Solvency and aromatic content can be relevant, and Group I is still used.

Greases and specialty applications

• Greases commonly use medium and heavy grades, including bright stock, as the oil phase.
• Marine lubricants, including cylinder oils, rely on bright stock and heavy Group I or II grades.
• Transformer and specialty oils have their own specifications, and the group is only one of several considerations.

The takeaway is that Group I is not obsolete: it remains important for heavy grades, greases and markets where finished-lubricant specifications permit it. Group II and III are growing in demand where tighter specifications apply. Your buying decision follows your customers' finished-product specification.

What to check on a base oil CoA

A CoA is the baseline document for any base oil purchase. Check these lines against your contract specification, and confirm the test method for each.

Test method numbers are given as common references. Your contract should state exactly which methods apply, and the CoA should name them.

Checks that catch problems

1. Viscosity against grade. Compare the 40°C and 100°C results to your specification, not just to the nominal grade name.
2. VI against group. A Group III offer needs a VI of 120 or higher. If the CoA says 118, it is not Group III, whatever the label says.
3. Sulphur and saturates. For Group II or III, both must meet the thresholds. A CoA that omits saturates or sulphur for a claimed Group II or III is incomplete.
4. Flash point. A low flash point against the grade suggests light fractions or contamination.
5. Colour. A sudden change in colour from a regular supplier can indicate a change in feedstock or process. Colour alone does not prove quality, but changes should prompt questions.
6. Origin and type. Ask whether the oil is virgin base oil from a named refinery, and whether it is the same source as earlier cargoes. If the offer could be re-refined base oil, it should say so; re-refined oils are legitimate but are a different product category and may be subject to different customer acceptance.
7. Batch and tank identification. The CoA should tie to the specific shipment tank or lot, with date and signatory.

Independent verification

For a first purchase from a new source, or for any large cargo, independent inspection is worth the cost. Pre-shipment inspection by SGS or Intertek, which Chemineral can arrange, covers sampling at loading, quantity and quality testing against the contract specification, and a report you can use to resolve claims.

Common buying mistakes

• Buying on group alone. Two Group II cargoes can differ in viscosity, colour and volatility. Specify numbers, not just the group.
• Treating "Group II+" or "Group III+" as a specification. These are marketing labels. Write the VI, volatility and colour you need into the contract.
• Accepting a typical-values data sheet as a CoA. A data sheet describes a product in general. A CoA describes the lot you are paying for.
• Ignoring the receiving tank. A clean cargo can be spoilt by a dirty tank, hose or flexitank at the destination. Agree who is responsible for cleanliness checks at discharge.
• Mixing grades without testing. Blending a new cargo into existing stock changes the viscosity of the whole tank. Test before you commingle.

Packaging, transport and handling

Base oils are generally traded as bulk liquids or packed in drums and tank containers.

• Bulk tankers. For large volumes between terminals with proper storage.
• ISO tanks and flexitanks. Common for mid-size parcels. Flexitanks are single-use bladders in a standard container; suitability depends on the oil, its temperature and the route, so confirm with the supplier.
• Drums. Steel or plastic drums, normally 200 litres, for smaller lots and for customers without tank storage.
• IBCs. Intermediate bulk containers for mid-size delivery.

Handling points that affect quality:

• Cleanliness. Tanks, hoses and flexitanks must be clean and free from previous cargo. Contamination with water, other oils or solvents causes disputes.
• Heating. Heavy grades such as bright stock are viscous at ambient temperature and may need heating to pump. Ask about loading and discharge temperatures.
• Safety. Base oils are generally not classified as dangerous goods for transport when their flash point is high, but classification depends on the specific product and its flash point. Check the SDS for the actual grade, and note that spills are slippery and an environmental hazard.

Commercial points worth agreeing up front

• Specification and tolerances. Spell out viscosity range, VI minimum, sulphur and saturates limits, flash point minimum and colour maximum.
• Quantity tolerance. Typically stated as a percentage at seller's or buyer's option. Agree it before you sign.
• Sampling and inspection. Who samples, at what point, and which results govern.
• Documents. CoA, SDS, certificate of origin, packing list and bill of lading.
• Routing. Chemineral quotes Suez and Cape routing side by side from Sohar and Salalah, so you can see the trade-off between transit time and freight cost for your destination.
• Offer validity. Firm offers from Chemineral are valid for 72 hours, which is the window to confirm specification and routing.

Frequently asked questions

Is Group II better than Group I?

For oxidation stability, low sulphur and modern engine-oil specifications, yes. But Group I is still the right choice for bright stock, certain greases and applications where its solvency and cost are advantages. "Better" depends on the finished product.

What is the difference between SN500 and Group II 500N?

Both are medium-viscosity grades, but they differ in composition and processing. SN500 is typically Group I; 500N is typically Group II. Compare measured viscosity, VI, sulphur and saturates, not names.

Can I use Group III in place of Group II?

Often yes, because Group III has higher VI and lower volatility, but it is usually more expensive and its solvency for additives can differ. Your formulator should confirm compatibility and test the blend.

What does a viscosity index of 95 tell me?

It means the oil changes viscosity with temperature to a moderate degree, typical of Group I and II oils (80 to below 120). It is not an indicator of poor quality by itself. The relevant question is whether it meets the specification for your finished product.

Do I need an SDS for base oil?

Yes. Request an SDS for every grade. It gives the classification, flash point, handling and spill guidance, and is required by most ports, carriers and customers.

How can I verify a Group claim?

Check that the CoA shows saturates, sulphur and VI, that each meets the API thresholds, and that the methods are stated. For high-value or first-time cargoes, arrange independent pre-shipment testing.

Next step: send us your enquiry

Chemineral is based in Sohar, Oman, and exports from Sohar and Salalah. If you are looking for base oil in a specific group and grade, send your specification, quantity, packing and destination port to our trade desk by submitting an RFQ through the contact page.