The basics about base stocks, including what they are, the difference between base stocks and base oils, and information on base oil groups.

Base stocks are the key building blocks of lubricants and greases. A base stock is a single lubricant component produced by a single manufacturer. ExxonMobil produces and supplies them, and oil marketers or formulators mix them to make the finished products.

The terms base stocks and base oils are often used interchangeably, but there are differences. A base stock is a single product, usually defined by its viscosity grade. A mixture of one or more base stocks in a finished lubricant is a base oil. A base oil is always defined in the context in the formulated lubricant. Base oil properties can vary depending on their API group.

The API classification system categorizes base stocks into numbered groups: Groups I, II, III, IV and V. The first three are refined from crude oil. There are a few Group III base stocks that are produced from bio-based feedstocks or natural gas. Group IV base stocks are fully synthetic polyalphaolefins, or PAOs. Group V is comprised of base stocks that do not fall into the other groups.

API base stock groups were first defined as a means to develop interchange guidelines in engine oils. The initial idea behind the base stock groups was that the manufacturing process would form a rough framework for considering interchange, thus originally, solvent refined base stocks were considered Group I, hydroprocessed base stocks were considered Group II, and all other base stocks, including chemically derived base stocks were considered Group III.

  Viscosity index (VI) Saturates Sulfur Notes
Group I 80-120 <90% >0.03% refined by separation processes, such as by removing aromatics by extraction or waxes by solvent-assisted crystallization
Group II 80-120 ≥90% ≤ 0.03% produced by hydrogen processing
Group III ≥120 ≥90% ≤ 0.03% produced by hydrogen processing
Group IV       polyalphaolefins (PAOs), fully synthetic
Group V       includes esters, certain bio-base stocks, naphthenic stocks, biolubes and all other base stocks that do not meet the Group I-IV definitions.
  • Group II have more saturates and less sulfur than Group I. They typically have better low temperature properties and are more oxidatively stable than Group I base stocks.
  • Group III base stocks have higher VI’s than Group I or II by definition and are even more oxidatively stable than Group II. Group III base stocks typically span a narrower range of viscosities than Group I or Group II.
  • Group IV is generally very oxidatively stable and has excellent low temperature performance. For these reasons, it is generally used only in the highest performing lubricants.
  • Group V are often blended with other base oils to affect the finished lubricant’s properties.


All base stock groups have properties that make them suitable in a specific range of applications.

While the API Group definitions provide a framework for classifying base stocks, base stocks within a specific group can have significantly differing properties, specifically viscosity index, low temperature viscosity, volatility, and saturates level. Not all base stocks within a given API Group can be expected to provide the same level of performance in a formulated lubricant. The most reliable method of predicting performance in field applications is by actual bench, rig and engine testing.

Paraffinic base stocks, and paraffinic base oils, are produced from paraffinic crude oil, which gives a good yield of stable base stocks with a high viscosity index. All ExxonMobil base stocks are paraffinic.

Naphthenic base stocks, and napthenic base oils, are produced from naphthenic crude. They are less stable and have a lower viscosity index than paraffinic base oils. However,they generally have outstanding low temperature performance characteristics. Transformer oils are often formulated with naphthenic base stocks.

There is no generally accepted definition of a synthetic base stock, or synthetic base oil. In the U.S., the government considers “synthetic” to be a marketing term characterizing the formulated lubricant while in Germany, synthetic base stocks are defined by law to be polyalphaolefins or esters. Many oil marketers consider lubricants formulated with significant concentrations of Group IV and/or Group III base stocks as synthetic. Most Group III base stocks are refined from crude oil streams.
Bright stock is a Group I base stock derived from deasphalted vacuum resid. It is used where high-viscosity or solvency is required. ExxonMobil offers a bright stock, CORE™ 2500, as part of its CORE™ base stock slate.

The API defines a base stock slate as a series of base stocks of the same group, from the same manufacturer, with different viscosities. Base stocks within a slate may be substituted for one another to achieve the same blended viscosity without the need for additional engine testing. For example, an oil marketer could substitute EHC™ 45 and EHC™ 65 for EHC 50in a qualified formulation without the need for engine testing. ExxonMobil offers CORE™ Group I and EHC™ Group II/II+, which are both identifiable as base stock slate under API guidelines.

Base oil interchange is a set of guidelines that allows the substitution of base stocks belonging to a specific base stock slate into a formulation approved with base stocks belonging to a different slate.
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