7 grades
API Group I, II and III stocks for lubricant blending, metalworking, grease manufacture and process use.
Base oil is most of the volume of any finished lubricant. API 1509 Annex E sorts every base stock into five groups on saturates content, sulphur content and viscosity index, and those groups are what an interchange decision hangs on.
Groups I, II and III all come from crude and differ only in how hard they were processed. Group IV is polyalphaolefin. Group V is everything else.
| Group | Saturates | Sulphur | Viscosity index |
|---|---|---|---|
| Group I | Below 90% | and / or above 0.03% | 80 up to below 120 |
| Group II | 90% or more | 0.03% or less | 80 up to below 120 |
| Group III | 90% or more | 0.03% or less | 120 or more |
| Group IV | Polyalphaolefins. Defined by chemistry, so no saturates, sulphur or viscosity index limit applies. | ||
| Group V | Everything not in Groups I to IV. Esters, polyalkylene glycols, naphthenics, alkylated naphthalenes and others. | ||
Test methods are fixed by the same annex: saturates by ASTM D2007 or D7419, viscosity index by ASTM D2270, and sulphur by ASTM D1552, D2622, D3120, D4294 or D4927. We supply Groups I, II and III.
This trips up more purchase orders than anything else on a base oil enquiry. Three different measurement bases are in circulation, and the digits look interchangeable when they are not.
Nominal viscosity in Saybolt Universal Seconds at 100 degrees Fahrenheit. It is not a viscosity at 40 or 100 degrees Celsius and it is not an ISO viscosity grade. Measured values often sit below the nominal figure, and different suppliers land in different places.
Nominal viscosity in Saybolt Universal Seconds at 210 degrees Fahrenheit. Same units as a solvent neutral number, different temperature, so a 150 bright stock and a 150 solvent neutral are nowhere near each other.
Nominal kinematic viscosity at 100 degrees Celsius in mm squared per second, measured by ASTM D445. These are trade shorthand rather than designations set by any standard, so confirm the group and the measured viscosity on the datasheet instead of reading them off the name.
Solvent refined stocks: vacuum distillation, then solvent extraction of aromatics and solvent dewaxing, without severe hydrogen processing. Group I covers a wider viscosity range than the other groups and dissolves additives and oxidation products readily, which is why blenders still buy it. It is also the only refining route that yields bright stock at all.
Bright stock. The heaviest paraffinic Group I stock, recovered from vacuum residue by deasphalting, solvent extraction, dewaxing and finishing. The name refers to the clear appearance once the wax is out.
Light paraffinic solvent neutral, SN 150. The lighter of our two neutrals, usually paired with a heavier grade in a blend.
Heavy paraffinic solvent neutral, SN 600. Roughly double the viscosity of a 150 neutral at 100 degrees Celsius, and the grade used to reach heavier finished viscosities before bright stock becomes necessary.
Hydrocracked and catalytically dewaxed, which converts or removes most of the aromatics, sulphur and nitrogen. Same viscosity index band as Group I but far cleaner, so it resists oxidation better at the same viscosity. Solvency is lower, so a switch from Group I is not a straight swap.
Nominal 3 mm squared per second at 100 degrees Celsius. The lightest grade in the BO series. At this viscosity the viscosity index usually lands below the Group III floor of 120, which is why it sits in Group II.
Severe hydrocracking plus catalytic hydroisomerisation and dewaxing, which rebuilds the wax into branched isoparaffins. Same purity limits as Group II with a higher viscosity index. Evaporation loss falls sharply as nominal viscosity rises across the series, which is the main lever for meeting volatility limits in modern engine oils. Base oils made from natural gas rather than crude also classify here.
Nominal 4 mm squared per second at 100 degrees Celsius, and the most widely traded Group III grade.
Nominal 6 mm squared per second at 100 degrees Celsius. Higher viscosity index than BO 4 and roughly half the evaporation loss, for jobs where volatility limits and film thickness matter more than cold flow.
Nominal 8 mm squared per second at 100 degrees Celsius, the heaviest of the commonly traded Group III grades. Lowest evaporation loss of the series. Group III refining does not extend much past this viscosity, and it yields no heavy neutral and no bright stock.
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