Base oil

What is meant by the term base oil?

Base oil is the main component of lubricants such as engine oils, transmission oils or greases. The type and quantity of base oils used determine the lubricating properties of the lubricant. Properties such as viscosity-temperature behaviour, oxidation resistance or friction behaviour are significantly influenced. Additives can be added to the lubricant to give it even more properties.

Oil drilling in a desert
Before we can develop our lubricants, crude oil must be extracted from the earth

Base oil groups

The API (American Petroleum Institute) has published a classification for the different base oils to make them more comparable. Aspects such as the refining method, the viscosity index, the proportion of saturated hydrocarbons and the sulphur content are taken into account.

Group Viscosity index (VI) Saturated hydrocarbons in % Sulphur content in % Description
I 80-120 < 90 > 0,03
  • Low treated mineral oils
  • Paraffins or naphthenes
  • Suitable for less demanding applications
II 80-120 ≥ 90 ≤ 0,03
  • More powerful mineral oils through hydrocracking
  • Pro: evaporation tendency, oxidation resistance, flash point
  • Contra: low temperature properties, pressure resistance
III > 120 ≥ 90 ≤ 0,03
  • Very powerful mineral oils due to strong hydrocracking
  • High stability and uniformity of molecules
  • Almost synthetic purity
IV - - -
  • Chemically constructed polyalphaolefins (PAO)
  • High performance synthetic lubricants
V - - -
  • Includes all base oils that do not belong to the first four classes.
  • Used for the production of additives
  • e.g. ester and polyester

Mineral oil vs. synthetic oil

In the case of mineral base oils (Group I-III), a low sulphur content in the oil is desired, since sulphur as an impurity in the oil can contribute to increased acid formation. The hydrocarbon compounds also require a high degree of saturation. This means that the compounds must be as closed as possible and no more free atoms can join. In practice, oxygen atoms usually accumulate at the free sites, which contributes to faster oil aging. In turn, a high viscosity index is important for the temperature resistance of the lubricant. The higher the VI, the better the oil can operate in low and high temperature ranges. This is particularly relevant for engine oils, as these must remain flowable at cold outside temperatures and high operating temperatures. Mineral oil is obtained exclusively from crude oil. It must be refined with separation and cleaning processes. The combination of the basic building blocks is random and depends on the respective crude oil source in which various foreign substances can be found.

The synthetic base oils from group IV and V have a high inherent VI, are saturated and sulphur-free. As a result, the oils are very resistant to ageing and temperature, but are also expensive to produce. Synthetic oil can also be obtained from crude oil, but also from coal or vegetable oils. Short hydrocarbon chains (C2H4 or C3H6) are combined to form lubricating oil molecules. Synthetic oil does not have to consist exclusively of hydrocarbons. It can contain oxygen (synthetic esters, polyglycols), phosphorus (phosphoric acid esters) or silicon (silicone oils). The molecular structure results from the machining process. The aim is to form homogeneous structures without undesirable foreign substances.

Advantages Disadvantages
Mineral oil
  • cheap production
  • large evaporation loss at high temperatures
  • bad cold behavior
  • single-grade oils with VI from 90-100 with limited application area
Sythetic oil
  • uniform composition
  • high aging stability
  • low evaporation loss
  • Multi-grade oils with VI from 130 to 150
  • good cold start behavior down to -40 °C
  • increased production costs through additional production steps


Heiko Stephan

Application Technology

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