A base oil is the refined oil that makes up most of a lubricant, before the additives are blended in. Engine oils, gear oils, hydraulic fluids and greases all start from one or more base oils. About 75 to 85% of the volume of a typical motor oil is base oil, and the remainder is the additive package.
What is a base oil made from?
Most base oil is made from crude oil. A petroleum base oil is produced from vacuum gas oil, the heavy distillate that comes off a refinery's vacuum column, by removing or converting the molecules that make a poor lubricant: aromatics, sulphur (US: sulfur) and nitrogen compounds, and wax. About 98% of base oils are derived from petroleum. The rest are synthetic, built up chemically from smaller molecules.
API 1509, the American Petroleum Institute's engine oil licensing standard, uses two terms. A base stock is a single lubricant component made by one manufacturer to one specification. A base oil is the base stock, or blend of base stocks, used in a finished oil. In trade the two words are used interchangeably.
Four properties describe how a base oil will behave:
- Viscosity. Resistance to flow, measured in centistokes (cSt) at 40°C and at 100°C.
- Viscosity index (VI). How much the viscosity changes with temperature. A high VI oil thins less as it heats up.
- Pour point. The lowest temperature at which the oil will still pour.
- Purity. The level of sulphur, nitrogen and aromatic compounds, usually expressed through the saturates content.
The API base oil groups
The API sorts all base stocks into five groups. The definitions are in API 1509, Annex E, and the first three groups are separated by three measured properties: saturates, sulphur and viscosity index.
| Group | Saturates | Sulphur | Viscosity index | How it is made |
|---|---|---|---|---|
| Group I | Less than 90% | and/or more than 0.03% | 80 or more, below 120 | Solvent extraction and solvent dewaxing |
| Group II | 90% or more | and 0.03% or less | 80 or more, below 120 | Hydrocracking and catalytic dewaxing |
| Group III | 90% or more | and 0.03% or less | 120 or more | Severe hydrocracking; also gas-to-liquids |
| Group IV | Not defined by these limits | Polyalphaolefins (PAO), synthesised | ||
| Group V | Not defined by these limits | All other base stocks not in Groups I to IV |
The "and/or" in the Group I line matters. A base stock falls into Group I if it fails either the saturates test or the sulphur test, provided its viscosity index is in the 80 to 120 band.
API 1509 also fixes the test methods: ASTM D2007 for saturates, ASTM D2270 for viscosity index, and one of ASTM D1552, D2622, D3120, D4294 or D4927 for sulphur. A certificate of analysis that reports those three numbers by those methods places a lot in its group.
The API introduced the group system in 1993. The terms Group II+ and Group III+ are also used in the market, for Group II stocks with a VI above about 110 and Group III stocks with a VI above about 130. They are marketing terms and are not API categories.
How each group is made
Group I is the oldest route. A selective solvent extracts much of the aromatic, sulphur and nitrogen content from the vacuum gas oil. A second solvent is then added and the mixture is chilled so that the wax crystallises and is filtered out. That wax is the feedstock for paraffin wax. Group I oils keep more sulphur and aromatics than hydroprocessed oils. They are widely used in industrial and marine lubricants and in engine oils for older engines.
Group II starts from the same feedstock and passes it through a hydrocracker at very high pressure, frequently above 2,000 pounds per square inch. Hydrogen saturates the aromatic molecules and converts sulphur to hydrogen sulphide and nitrogen to ammonia. A catalytic dewaxing unit then converts the wax molecules to branched ones instead of removing them. The result is a clear, very pure oil, typically about 99% saturates.
Group III is made the same way under more severe hydrocracking conditions, which raises the viscosity index to 120 or more. Oils made from natural gas by the gas-to-liquids route are also classed as Group III. In many markets Group III oils are sold as synthetic.
Group IV covers polyalphaolefins only. PAOs are synthesised from smaller olefin molecules and perform well at very high and very low temperatures.
Group V is everything else: naphthenic oils, esters, polyalkylene glycols, phosphate esters, silicone oils and alkylated aromatics.
One further division cuts across the mineral groups. Most base oils are paraffinic, made from paraffinic crudes. About 15% of the world's base oils are naphthenic, also called pale oils, which contain more ring-shaped molecules and very little wax.
What do SN150, SN500 and bright stock mean?
Group I base oils are traded by viscosity grade, under names that predate the API groups.
- SN stands for solvent neutral: a solvent-refined distillate base oil. The number that follows is a viscosity grade. The higher the number, the heavier the oil.
- SN150 is a light neutral. As an example, the ORLEN refinery's SN 150 sheet gives a kinematic viscosity of 28.8 to 33.5 cSt at 40°C and 5.0 to 5.5 cSt at 100°C, a viscosity index of 95 minimum, a flash point of 210°C minimum and a pour point of -12°C maximum.
- SN500 is a heavy neutral, much more viscous than SN150 at the same temperature.
- Bright stock is the heaviest Group I grade. It is not a distillate. It is made from deasphalted vacuum residue, the bottom of the vacuum column with the asphalt removed.
A blender uses these as a ladder: a light neutral to thin a blend, a heavy neutral for body, and bright stock where a very high viscosity is needed. See SN150 base oil, SN500 base oil and bright stock.
Viscosity numbers differ between producers, so a grade name is not a specification. An enquiry should state the kinematic viscosity at 40°C or 100°C that the blend needs.
What is base oil used for?
- Automotive lubricants. Engine oils, transmission fluids and gear oils. Modern low-viscosity engine oils rely on Group II and Group III.
- Industrial lubricants. Hydraulic, turbine, compressor and circulating oils.
- Marine lubricants. Cylinder and system oils for ship engines.
- Greases. Base oil thickened with a soap or other thickener.
- Process oils. Rubber extender oils, transformer oils and carriers for chemicals.
- White oils. Base oil refined further until it is colourless and close to inert, for pharmaceutical, cosmetic and food uses. See white mineral oil.
What a base oil specification covers
A producer's sheet for a base oil lists kinematic viscosity at 40°C and 100°C, viscosity index, flash point, pour point, colour, sulphur and saturates, and often Noack volatility and carbon residue. For the group itself, the three API numbers decide: saturates, sulphur and viscosity index.
The group pages on this site set out the producer specifications we work from: Group I, Group II and Group III.
How base oil is traded
Base oil is sold per tonne, in bulk by chemical or product tanker, in flexitanks and ISO tanks, or in drums and IBCs. Terms are FOB, CFR or CIF. The contract names the grade, the producing refinery and the specification, and cargo tanks must be clean enough to protect colour and water content.
CBIL is a commodity brokerage company. We broker base oil between a named refinery or seller and a named buyer, against independent inspection by SGS, Intertek or a similar company at loading. The grades are listed on our base oil page.
Request a base oil quote with the group or grade, viscosity, quantity, packing and destination port.