Base oil is the liquid that makes up most of a lubricant; additives, in smaller amounts, give it its final function. It comes from crude oil refining or from chemical synthesis, and the API (American Petroleum Institute) sorts it into five groups, I to V, by three measurements: saturates, sulfur and viscosity index. To choose: for lubricants, the group decides (II and III for stability and long service life); for rubber, paint and adhesives, the chemical family decides: paraffinic, naphthenic or aromatic.
Think of the flour in a loaf of bread. It is almost all of the dough and sets the texture; yeast and salt go in small amounts and change the result, but they cannot fix bad flour. In a lubricant, base oil is the flour and additives are the yeast. The three API measurements tell you how that flour behaves: saturates are molecules with no double bonds and no aromatic rings, so they resist oxygen better; sulfur is a leftover from crude oil that shows how thoroughly refining cleaned the base; viscosity index shows how much the oil thins when hot and thickens when cold (the higher the number, the less it changes).
Which base oil should you use for each application?
Start with what your plant makes. If you blend high-performance lubricants, you choose by stability, and that points to Groups II and III. If you make rubber, paint, adhesives or textile fibers, you choose by chemical family: paraffinic, naphthenic or aromatic. The table summarizes the technical data sheet of each line of base oils for blenders and chemical plants.
| If your plant… | Use | Why |
|---|---|---|
| Blends high-performance or low-viscosity engine oil | HBP-E (Group III+, a market name for higher-index Group III) or the HBP-S line (Group III) | High viscosity index and low volatility (little oil lost to evaporation when hot) |
| Blends industrial lubricants, hydraulic oil, transmission fluid, turbine oil or grease | HBP-I line (Group II) | Hydrorefined (treated with hydrogen): less sulfur and fewer aromatics, better oxidation stability and a high flash point (the temperature at which the oil vapor ignites for an instant when a flame passes near it) |
| Makes rubber goods, treats textile fibers, produces oil-based paint, leather softeners or temporary protectives, or needs an additive-free oil | HBP line (paraffinic) | Virgin paraffinic oils (refined straight from crude, with no re-refined oil), made to plasticize (soften) and extend (add volume to the compound) elastomers, that is, rubbers, and to serve as a base for oils and greases |
| Makes light-colored rubber, adhesives, low-temperature lubricants or soluble cutting fluids | HBN line (naphthenic) | Flows in the cold (low pour point) and blends well with rubber |
| Makes tires or elastomer compounds that need a middle ground between paraffinic and naphthenic | HPN (mixed base) | A blend of virgin base oils, with a composition between paraffinic and naphthenic |
| Makes dark rubber, such as polybutadiene (BR) and styrene-butadiene (SBR), or printing ink | HBA (aromatic) | Aromatic oil derived from crude oil residue, used as extender and plasticizer |
Each line comes in several grades, from the thinnest to the most viscous. The grade follows from the viscosity your formulation needs, and the Kelpen Oil Technical Team checks the choice against the data sheet of that grade.
What is base oil made of?
Mineral base oil is made from the heavy fractions of crude oil, after removing what oxidizes and what hardens in the cold. There are five steps, and the way the cleanup is done (with solvent or with hydrogen) decides the group:
Atmospheric distillation. Heated crude separates by boiling temperature: gas, gasoline, kerosene and diesel leave from the top and the side draws of the column, and a heavy liquid stays at the bottom, the atmospheric residue.
Vacuum distillation. At low pressure, that residue boils at a lower temperature, without breaking down. The oil cuts come out, separated by boiling range, which gives them different viscosities (spindle, the thinnest, and the light, medium and heavy neutrals), and the vacuum residue is left over.
Deasphalting. A solvent (propane) separates the asphalt from the vacuum residue and recovers the deasphalted oil, which is very viscous and becomes bright stock, the most viscous base oil from the refinery, used in gear oils and in oils for slow, heavily loaded machines.
Aromatics removal. Aromatics are ring-shaped molecules that lower the viscosity index and, when they oxidize, darken the oil and form sludge (a pasty deposit of oxidized oil). On the solvent route, they are extracted and leave as aromatic extract. On the hydrogen route, hydrotreating or hydrocracking (hydrogen, catalyst, pressure and temperature) turns aromatics into saturates and removes sulfur and nitrogen as gas, which leads to a more stable base.
Wax removal. Wax is made of straight-chain molecules that crystallize in the cold and cloud the oil. On the solvent route, the wax is crystallized at low temperature and leaves as paraffin. On the hydrogen route, catalytic dewaxing, usually by isomerization (rearranging a straight molecule into a branched one of the same size), turns the wax into isoparaffin, a branched molecule that stays liquid in the cold.
In the end, the solvent route gives Group I. The hydrogen route usually gives Group II when the treatment is moderate and Group III when it is severe, with a viscosity index of 120 or more. A final hydrogen polish (hydrofinishing) can be added on both routes to lighten and stabilize the base.
How this shows up in your plant: the route leaves visible signs. Group II and III base oils are almost colorless, Group I ranges from light to amber depending on the cut, and aromatic oil is dark. Wax left in the oil shows up as haze or a whitish deposit in the drum on a cold day. And in equipment that runs hot, a base with more aromatics darkens and forms sludge sooner, which shortens the drain interval.
Figure 1. From crude oil to base oil: the five refining steps and where Groups I, II and III split. Each cut goes through steps 4 and 5 separately; the red arrows show what leaves at each step, and naphthenic oils and Groups IV and V follow a different path.
What are the base oil groups?
The API classification has five: Groups I, II and III are mineral and are separated by saturates, sulfur and viscosity index; Group IV is polyalphaolefin (PAO), a synthetic; Group V covers every other base.
| Group | How it is made | Saturates (% by mass) | Sulfur (% by mass) | Viscosity index |
|---|---|---|---|---|
| I | Solvent route | Below 90 | Above 0.03 | 80 to less than 120 |
| II | Hydrogen route, moderate treatment | 90 or more | Up to 0.03 | 80 to less than 120 |
| III | Hydrogen route, severe treatment or wax isomerization | 90 or more | Up to 0.03 | 120 or more |
| IV | Chemical synthesis: PAO, built from small molecules | Defined by chemistry, with no numeric limit | ||
| V | All others: esters (made from acids and alcohols), PAG (polyalkylene glycol, a synthetic made from ethylene and propylene oxide), silicones, naphthenics and more | Everything that does not fit Groups I to IV | ||
Under the API rules, missing either of the two chemical limits is enough to put a base in Group I: saturates below 90% or sulfur above 0.03%. Group III+ is a market name, with no definition from the API or from Brazil's regulator, for Group III with a higher viscosity index and lower evaporation loss, made with more severe hydrocracking and wax isomerization. In the official table it is still Group III. Where switching from Group I to Group II pays off, and where it does not, is covered in the article on Group II base oil.
In Brazil, base oil sales are regulated by the ANP (National Agency of Petroleum, Natural Gas and Biofuels), through Resolution No. 911/2022. It uses the same API limits for Groups I, II and III and keeps a separate class, naphthenic, defined by where the base comes from: the crude behind it must be naphthenic or intermediate. That is measured by the KUOP factor, a number calculated with the UOP 375 test method that shows whether a crude leans paraffinic or naphthenic (the higher the number, the more paraffinic the crude); the naphthenic class requires a KUOP above 10 and below 12.5. In the API table, naphthenic oils usually fall into Group V, because their viscosity index is below 80.
On the label of a finished lubricant, the word synthetic usually means a Group III, IV or V base; mineral means Group I, II or naphthenic.
What is the difference between base oil and lubricant?
Base oil is the raw material; lubricating oil is the finished product, made of base oil plus an additive package chosen for the machine. According to the ANP, the most common additives are detergents, dispersants, antioxidants, antifoams, viscosity index improvers and pour point depressants. Grease follows the same logic: a thickener, usually a metallic soap, dispersed in base oil.
Each part is responsible for something different:
The base decides how well the oil resists oxygen and heat, how much it evaporates, how it behaves in the cold and how much additive and rubber it dissolves.
The additive decides the specific job: protecting against wear, carrying the load between gear teeth, keeping an engine clean, breaking foam.
That is why two hydraulic oils with the same viscosity grade and the same additive can last for different lengths of time in the same machine: the difference is the base. A viscosity index improver makes up for part of a low-index base, but it is a polymer that can break down under shear (the cutting stress the oil suffers between moving parts), and then viscosity drops with use.
What is base oil used for?
It is used to make lubricants and greases and, without additives, as a raw material for other industries. The data sheets of the HBP, HBN, HPN and HBA lines list these uses:
rubber and tires, as plasticizer and extender (oils that soften the compound and make it easier to process);
oil-based paints, printing inks and adhesives;
textile fiber treatment and leather softeners;
temporary protectives and soluble cutting fluids;
petroleum jelly, release pastes and raw material for agricultural products.
In these uses the chemical family decides, and it is read in the carbon-type composition (the share of carbon that sits in paraffinic chains, naphthenic rings and aromatic rings): paraffinic (straight and branched chains, stable), naphthenic (saturated rings, flows in the cold and blends well with light-colored rubber) or aromatic (aromatic rings, with strong dissolving power, for dark rubber). How to choose among the three by carbon-type composition and aniline point (a test that measures how strongly the base dissolves) is covered in the article paraffinic, naphthenic or aromatic oil.
What happens to used lubricating oil?
It must be collected and re-refined. In Brazil, CONAMA Resolution No. 362/2005, from the National Environment Council, requires all collected used oil to go to re-refining, which turns it back into base oil; burning or incinerating used oil does not count as recycling or as proper disposal.
Re-refining puts used oil through steps similar to those of refining to remove water, fuel, metals, spent additives and oxidation products. According to the ANP overview published in 2023, re-refined base oils account for about 20% of the base oils used in Brazil. ANP Resolution No. 911/2022 sets the specification for re-refined Group I in its own table, separate from the table for virgin base oils (those made straight from crude).
To know what you are buying, check the paperwork: every load of base oil sold in Brazil must come with a legible certificate of quality, and the invoice shows the product's ANP code and the number of that certificate. The Kelpen Oil HBP, HBP-I, HBN and HPN lines are virgin base oils, as their data sheets state.
How does Kelpen Oil check each batch of base oil?
Every batch is tested in the in-house laboratory in Arujá (SP) and ships with a certificate of analysis; one result out of range keeps the batch from leaving the plant. The Kelpen Oil Technical Team checks each batch by measurement, never by the product name or by viscosity alone: two base oils of the same grade can belong to different groups or families, and the viscosity index and the carbon-type composition show that difference. The standards in the table are test methods published by ASTM (the American standards body) and by ABNT, the Brazilian standards association, whose standards carry the prefix NBR.
| What we measure | What it reveals | Standard and instrument |
|---|---|---|
| Viscosity at 40 °C and 100 °C | The grade of the base oil and the two points used to calculate the viscosity index | ASTM D7042, SVM 4001 viscometer |
| Viscosity index | How much viscosity changes with temperature; separates Group III (120 or more) from Groups I and II | NBR 14358, SVM 4001 |
| Carbon-type composition (CA, CN, CP) | The percentage of aromatic, naphthenic and paraffinic carbon: the family of the base and how much aromatic content refining left behind | ASTM D2140 and D3238, with refractive index (how much light bends as it passes through the oil) on the Abbemat 350 refractometer and density and viscosity on the SVM 4001 |
| Sulfur content | In high-sulfur bases, such as aromatic oil and Group I, it confirms batch-to-batch consistency; the method was designed for additive elements and does not decide the group | X-ray fluorescence, ASTM D6481 |
| Aniline point | Dissolving power (solvency): the lower it is, the more the base dissolves rubber and additives | ASTM D611 |
| Pour point | The lowest temperature at which the oil still flows; reveals wax left in the base | NBR 11349 |
| Color | The degree of refining and batch-to-batch consistency | NBR 14483 |
| Infrared spectrum | The chemical fingerprint of the batch, compared with the product standard | Comparison with the standard, based on ASTM E2412 |
Saturates content, the third API criterion, has its own method (ASTM D2007) and is not among the laboratory's 25 tests. For that reason, the group of the HBP-I (Group II), HBP-S (Group III) and HBP-E (Group III+) lines is the one stated on each data sheet; for the lines chosen by family (HBP, HBN, HPN and HBA), carbon-type composition and aniline point check the batch. The full list, with standard and instrument, is in the article on the lubricants laboratory tests.
Frequently asked questions
Can base oil be used on its own, without additives? Yes, where the machine or the process does not call for additives. The data sheets list HBN grades for reciprocating (piston) air compressors with low air discharge temperature, except refrigeration compressors, and for oil-bath bearings, circulating systems and gearboxes that do not require an additive oil, always when the equipment manufacturer recommends it.
What is SN 150 base oil? SN stands for solvent neutral: a paraffinic cut from vacuum distillation refined by the solvent route, and neutral oils are graded light, medium and heavy by viscosity, with spindle as the thinnest cut. The number, as in SN 150, is the approximate viscosity in Saybolt seconds (SUS, an old unit that measures viscosity by the time the oil takes to flow through an orifice) at 100 °F, or 37.8 °C.
Does Brazil produce Group III base oil? According to the ANP overview from 2023, Brazil produces only Group I and Group II base oils and naphthenics. The Group III used in the country is imported, as is a large share of the base oil consumed in Brazil.
Glossary
Saturates: hydrocarbon molecules with no double bonds and no aromatic rings, more resistant to oxygen.
Viscosity index (VI): a number that shows how much viscosity changes with temperature; the higher it is, the less the oil changes.
Hydrorefining: treatment with hydrogen, catalyst, pressure and temperature that turns aromatics into saturates and removes sulfur and nitrogen.
Dewaxing: the step that removes wax or turns it into isoparaffin, so the oil does not harden in the cold.
Aromatic extract: the aromatic-rich fraction removed on the solvent route, used as a process oil for rubber.
Aniline point: the temperature at which equal volumes of oil and aniline mix completely; it indicates the dissolving power of the base.
PAO (polyalphaolefin): the Group IV synthetic base, built by chemical synthesis.
Re-refining: the process that turns used lubricating oil back into base oil.
Talk to the Technical Team
The Kelpen Oil Technical Team reviews what your formulation or process requires (stability, solvency, cold behavior, viscosity and color) and confirms the right base oil. If you want to check a base oil you have already bought, or if the oil is already in service, the laboratory in Arujá (SP) runs the oil analysis as a paid service: viscosity, viscosity index, color, carbon-type composition and aniline point are among the 25 tests on the list. Save this article or send it to whoever buys raw materials at your company.
