Group II base oil is a hydroprocessed mineral oil, that is, one treated with hydrogen. Under API 1509, the American Petroleum Institute publication, it has at most 0.03% sulfur and 90% or more saturated molecules, which are filled with hydrogen and resist oxygen better. Switch from Group I to Group II when the oil runs hot, with air, or for a long time between changes. Keep Group I or naphthenic base oil (another base oil family) when the application needs solvency: dissolving additives and penetrating rubber.
Base oil is most of a lubricant formulation; additives go on top of it, the way yeast goes into flour for bread. The group tells you what molecules that flour is made of, and that matters for how well it resists heat and the oxygen in the air.
Group I or Group II: which base oil should you use?
Use Group II when the lubricant has to last under heat and air; use Group I or naphthenic when the formulation depends on solvency. The table sums up the most common cases for anyone formulating a lubricant, a grease or a rubber compound.
| If, at your plant... | Use | Why |
|---|---|---|
| You formulate a hydraulic, turbine or circulation oil (the oil a pump keeps sending through the bearings and back to the tank) that runs hot and stays in the equipment for a long time | Group II, HBP-I line | With the right antioxidant, it oxidizes more slowly and forms less sludge |
| You formulate engine oil for API (American Petroleum Institute), ACEA (association of European car makers) or ILSAC (the fuel-economy standard of American and Japanese automakers) requirements | Group II (HBP-22 I PLUS, HBP-30 I, HBP-40 I, HBP-50 I) or Group III | The bulletins for these grades say that controlled volatility and viscosity index make it possible to formulate to meet or exceed these requirements |
| You make grease with long relubrication intervals | Group II, HBP-I line | The HBP-I line bulletins mention greases, and the more stable base helps when the grease stays in the bearing for a long time |
| You make a rubber compound (with a plasticizer or extender, the oil that softens rubber and adds volume to the compound), oil-based paint, or treat textile fiber and need solvency | Group I, HBP line, or naphthenic bases | Aromatics (and naphthenic bases, as a family) dissolve better and penetrate rubber better. Exception: the bulletins for HBP-30 I, HBP-40 I and HBP-50 I, all Group II, provide for all three uses |
| Your Group I formula is approved and has no oxidation problem | Keep the current base | Switching means redoing the oxidation, sealing and additive solubility tests |
What are Group II base oils?
They are paraffinic mineral base oils that went through hydroprocessing and meet the three limits of API 1509 (Appendix E), the American Petroleum Institute publication that sorts base oils into five groups: 90% or more saturates, at most 0.03% sulfur and a viscosity index of 80 to 119. The viscosity index (VI) tells how much the oil thins when it heats up: the higher the number, the less the viscosity changes. In Brazil, ANP Resolution 911/2022, from the national petroleum agency, uses the same limits for Group II and requires every base oil shipment to come with a quality certificate.
Paraffinic means that most of the carbon atoms form chains, like the paraffin in a candle. Hydroprocessing is treatment with hydrogen, under pressure and temperature, that removes sulfur and nitrogen and saturates the aromatic rings. What is left is a more uniform molecular structure, and that uniformity gives repeatable behavior from batch to batch.
With the same saturates and sulfur limits and a VI of 120 or more, the base is already Group III (at Kelpen Oil, the HBP-S line); part of the market sells Group III as synthetic, even though it also comes from petroleum. At Kelpen Oil, a Brazilian lubricant manufacturer since 1999 and a supplier of base oils for lubricant producers, Group II is the HBP-I line. The grades run from HBP-10 I to HBP-120 I: from the light fluid used in greases and industrial lubricants to bases for automotive and industrial formulation.
What is the difference between Group I and Group II?
The difference is in the process and the chemistry. Group I is solvent refined: the solvent washes the petroleum distillate and carries away only part of the aromatics, so it keeps more aromatics and sulfur. Group II is hydroprocessed and ends up with 90% or more saturates and up to 0.03% sulfur. The viscosity index can be the same in both.
| Criterion | Group I | Group II |
|---|---|---|
| Process | Solvent refining | Hydroprocessing |
| Saturates (% by mass) | Below 90% | 90% or more |
| Sulfur (% by mass) | Above 0.03% (300 ppm) | Up to 0.03% (300 ppm) |
| Viscosity index | 80 to 119 | 80 to 119 |
| Oxidation stability | Reference; part of the sulfur acts as a natural antioxidant | Superior, when formulated with the right antioxidant |
| Solvency (response to additives and rubber) | Higher | Lower |
| Color | Darker | Lighter |
To be Group I, it is enough for one of the two chemical criteria to fall outside the limit: saturates below 90% or sulfur above 0.03%.
In the table, the row that decides is oxidation stability, because viscosity can be the same in both groups. Oxidation is the reaction of the oil with oxygen from the air, sped up by heat. Two base oils with the same ISO VG (the viscosity grade at 40 °C) and different groups age at different speeds inside the equipment, in a predictable sequence:
multi-ring aromatics and nitrogen compounds make it easier for oxygen to attack;
oxidation raises acidity, measured as the total acid number (TAN), and forms sludge, a mud of oxidation products;
the sludge settles in oil passages, filters and valves and, under heat, hardens into varnish;
the deposit brings the oil change forward, a cost that does not show in the price per liter.
One detail changes the decision. Part of the sulfur compounds in Group I work as a natural antioxidant. Group II, with almost no sulfur, depends on the antioxidant the formulator adds: with the right package it lasts longer than Group I in oxidation tests; with a package designed for Group I, it may perform below expectations.
Group I has another asset that hydroprocessing reduces: solvency. The aromatics that make oxidation easier are the same ones that dissolve certain additive packages better and interact better with elastomers (rubbers). That is why rubber plasticizers and extenders usually use Group I or naphthenic bases. The rule has an exception: the bulletins for HBP-30 I, HBP-40 I and HBP-50 I, all Group II, provide for use as a plasticizer and extender, in oil-based paint and in textile fiber treatment, and the compound test decides.
What does hydroprocessing change in the oil?
Hydroprocessing swaps impurities for hydrogen: it takes sulfur and nitrogen out of the molecules and saturates the aromatic rings, which leaves the base lighter in color and, with the right antioxidant, more resistant to oxidation. The sequence inside the refinery:
The petroleum distillate enters a reactor with hydrogen (H₂) and a catalyst, under high pressure and temperature.
The hydrogen breaks the bond between sulfur or nitrogen and carbon; they leave as gas, in the form of hydrogen sulfide (H₂S) and ammonia (NH₃), which brings sulfur down to 0.03% (300 ppm) or less.
The aromatic rings take up hydrogen and become saturated (naphthenic) rings, which brings the saturates content to 90% or more.
With fewer aromatics and almost no heteroatoms (sulfur, nitrogen and oxygen atoms attached to the molecule), the oil offers fewer points of attack to oxygen and darkens less.
In practice, the refinery may combine hydrocracking (which also breaks and rearranges molecules), dewaxing (which removes the wax that would stiffen the oil in the cold) and hydrofinishing (a last light hydrogen treatment that stabilizes color) to reach Group II. For the buyer, what counts is the measured result: saturates, sulfur and VI. The VI stays in the 80 to 119 range; at 120 or more, the base would be Group III.
How this shows up at your plant: a Group II base arrives lighter in color, and the oil formulated with it darkens and acidifies more slowly in service. With Group I in a hot cycle, the signs come sooner: a clogging filter, varnish on valves, TAN rising in the analysis and an early oil change. For the formulator, a badly done switch has another sign: additive clouding in the cold or seals shrinking, because the new base dissolves and swells rubber less.
Figure 1. Simplified hydroprocessing: with hydrogen and catalyst, sulfur (S) and nitrogen (N) become gas (H₂S and NH₃), which leaves the reactor mixed with the oil and is separated from it later, and the aromatic ring becomes a saturated ring; the side chains stay the same. Below, the API 1509 limits for Group II: saturates of 90% or more, sulfur up to 0.03% (300 ppm) and VI of 80 to 119; with a VI of 120 or more, the base is Group III.
When does switching from Group I to Group II pay off?
It pays off when the formulation runs hot, aerated or for long intervals. Severe-cycle hydraulic oils (the guide to types of hydraulic oil shows the classes), industrial circulation lubricants, long-life greases and automotive formulations chasing modern specifications benefit directly from the extra stability. Recent engine oil categories limit volatility and ask for more oxidation stability, which moved formulators to Groups II and III.
It does not pay off when solvency is the requirement. Rubber plasticizers, extenders, adhesive formulations and applications where the base has to "carry" the additive well are still better served by Group I or naphthenic, except for the Group II grades that the bulletin already provides for rubber (HBP-30 I, HBP-40 I and HBP-50 I). Switching for the sake of switching, in these cases, means paying more for a property the application does not use.
When the switch pays off, it calls for four checks before going into production:
adjust the antioxidant package for the low-sulfur base;
test additive solubility, including in the cold, because the new base dissolves less;
check the equipment seals, which swell less with fewer aromatics;
repeat the oxidation test and the viscosity of the finished product, to confirm the gain.
The criterion, in short: stability calls for Group II; solvency calls for Group I or naphthenic.
What is the difference between C2 and C3 oil?
C2 and C3 are ACEA categories, from the European association of car makers, for finished engine oil; they sit outside the base oil classification. Both are in the intermediate SAPS range (Mid SAPS, for sulfated ash, phosphorus and sulfur), with at most 0.8% sulfated ash. Ash is the mineral residue that clogs the particulate filter; phosphorus and sulfur poison the exhaust catalyst. The main difference is the viscosity measured at 150 °C while the oil is sheared, that is, squeezed between parts very close together, as in the crankshaft bearing (HTHS, in mPa·s, the millipascal-second, a unit of dynamic viscosity; for comparison, water at 20 °C is about 1 mPa·s). The minimum is 2.9 mPa·s for C2 and 3.5 mPa·s for C3. C2 has a thinner film and saves fuel; C3 protects more under high load.
The confusion is common because "Group II" and "C2" sound alike. The rule to avoid mistakes: Group (I to V) classifies the raw material; C2 and C3 classify the finished lubricant. A C3 oil can be formulated on Group II or III bases, and the number of one has no relation to the number of the other.
How do you tell which group your base oil is?
The analysis report answers: compare sulfur, saturates and viscosity index with the API 1509 limits. The place to look for these numbers is the quality certificate that ANP requires on every shipment. Three points when reading it:
Sulfur in ppm: the report usually gives sulfur in parts per million (ppm). Since 0.03% equals 300 ppm, a Group II must show 300 ppm or less.
Saturates or aromatic carbon: the saturates content has its own test (ASTM D2007, from ASTM, the American standards body for test methods). When the report does not give saturates, it may give the carbon distribution: the percentage of carbon atoms in aromatic rings (CA), in saturated rings (CN) and in chains (CP). Low aromatic carbon points to a hydroprocessed base, but API 1509 classifies by saturates; to settle the group, ask for the saturates content in the certificate.
VI: the 80 to 119 range holds for Group I and Group II, so VI alone does not tell them apart; it only separates Group III, at 120 or more.
At its in-house laboratory in Arujá (SP), Kelpen Oil runs tests for viscosity index, for sulfur content and for carbon distribution, which show whether one batch repeats the other. The standards in the table are ASTM or NBR (Brazilian standard from ABNT). No batch leaves the plant without laboratory approval, and every batch has a report. For a formulator, the question "which group is it?" always comes with another: "is it the same as last month's?". The answer depends on measuring batch by batch. The method of each test is in the post on the 25 laboratory tests.
| What we measure | What it shows | Standard |
|---|---|---|
| Sulfur content, by X-ray fluorescence (Épsilon 1, Malvern Panalytical) | Sulfur content of the batch, to compare one batch with the previous one; the 300 ppm limit of Group II is confirmed in the producer's quality certificate | ASTM D6481 |
| Aromatic, naphthenic and paraffinic carbon (CA, CN and CP), calculated from the refractive index measured with the Abbemat 350 refractometer, density and viscosity | Whether the structure is that of a hydroprocessed base, with few aromatic rings | ASTM D2140 and ASTM D3238 |
| Viscosity index, on the SVM 4001 viscometer | Whether the VI is in the 80 to 119 range of Groups I and II or reaches 120, as in Group III | NBR 14358 |
| Viscosity at 40 °C and 100 °C, on the SVM 4001 | Whether the grade matches the order; both values go into the VI calculation | ASTM D7042 |
| Color, on the Lovibond PFX-i colorimeter | A hydroprocessed base comes out lighter; darkening points to oxidation | NBR 14483 (ASTM D1500) |
| Barium, boron, calcium, phosphorus and zinc, by atomic emission spectrometry with plasma (ICP, inductively coupled plasma; Optima 7000 DV, PerkinElmer) | Whether the base arrived pure, with no contamination by additive elements | NBR 14786 |
Frequently asked questions
Does Group II replace Group I in every formulation? No. It replaces it with an advantage where oxidation stability decides, as long as the antioxidant package is adjusted. Where solvency decides, as in plasticizers and adhesives, Group I or naphthenic remain the choice.
What does SN oil mean? SN stands for "solvent neutral", the traditional name of solvent-refined base oils, followed by the viscosity (SN 150, SN 500). By refining route, an SN is usually Group I; the sulfur and saturates in the report confirm it.
Is Group II base oil mineral or synthetic? Mineral. It comes from petroleum; hydroprocessing changes the purity and the structure of the molecules, and the origin stays the same. The first synthetic base in the classification is Group IV, polyalphaolefin (PAO).
Can Group I and Group II base oil be mixed? Yes: both are mineral hydrocarbons and they blend. The blend has intermediate properties and, if sulfur goes above 0.03% or saturates fall below 90%, it is no longer Group II; the report of the blend is what counts.
Does Kelpen Oil supply Group II base oil? Yes. Kelpen Oil supplies the HBP-I line, from HBP-10 I to HBP-120 I: a first-refining base oil (virgin oil, not re-refined), hydroprocessed at the source, with control by the in-house laboratory in Arujá (SP) and a report for every batch.
Glossary
Base oil: most of a lubricant formulation, before the additives.
API 1509: American Petroleum Institute publication that, in Appendix E, sorts base oils into five groups.
Hydroprocessing: treatment with hydrogen, catalyst, pressure and temperature that removes sulfur and nitrogen and saturates aromatics.
Saturates: molecules that already took all the hydrogen they can and resist oxygen better; Group II has 90% or more.
Aromatics: molecules with a benzene ring, which dissolve well and oxidize more easily.
Viscosity index (VI): how much viscosity changes with temperature; the higher the number, the less it changes.
Solvency: the ability of the oil to dissolve additives and to penetrate rubber.
ppm: parts per million; 0.03% equals 300 ppm.
Talk to the Technical Team
Moving between groups affects solvency, additive response and elastomer compatibility. Before you decide, the Kelpen Oil Technical Team evaluates the application, the viscosity, the additive package and the stability requirement, and confirms the right product. If you already use a base oil and want to know exactly what it is, the Arujá (SP) laboratory does the oil analysis for any company, including oil that did not come from Kelpen Oil, as a paid service. Save this article for your next base oil quote or send it to whoever formulates at your company.
