Base Oils

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Group I - Solvent Refined Mineral Base Oil:

Group 1 base oils are the least refined of all the groups. They are usually a mix of different hydrocarbon chains with little or no uniformity. While some automotive oils on the market continue to use Group I stocks, they are generally used in less demanding applications.

Modern mineral base oils are the result of a long and complex distillation and refining processes. The feedstock used is crude oil. This substance is not of uniform quality but consists of several thousands of hydrocarbon compounds in which the elements carbon and hydrogen are present in all molecules and, in part, are bound to other elements.

The hydrocarbons can be divided into three main groups: paraffinic, naphthenic and aromatic. 

Paraffinic hydrocarbons can be further divided into two subgroups: normal paraffinic and isoparaffinic. Paraffinic hydrocarbons are the best lubricants. The distillation process in the refinery separates the hydrocarbons contained in the crude into cuts based on the molecule size.

Furthermore, as many unwanted substances as possible are removed in the process, such as sulphur, aromatic hydrocarbons, paraffin wax, etc. In other words the mineral oil production process is physical cleaning and the end product is so-called paraffinic base oil.

Most of the hydrocarbons in the base oil are paraffinic, but it also contains naphthenic and aromatic molecules. When the finished lubricant, such as motor oil, is made of these, several additive compounds are used to improve the base oil properties.

The final outcome can also be so-called naphthenic base oil, where most of the hydrocarbons are naphthenic.  

Group II - Hydrocracked Base Oils

Base oils made by hydrocracking and isomerization technologies had such a signifigant increase in desirable performance over solvent refining technology that in 1993 the API categorized base oils by composition.    Solvent refined oils are now referred to as group I base oils.

Group II base oils are a vast improvement over group I because they contain lower levels of impurities and because they are so pure, they have almost no color at all.

Improved purity means the base oil and additives can last longer under use. The oil is more inert and forms less oxidation byproducts that can increase viscosity and react with additives.

Group II Base Oils over Group I Base Oils offer;

     * Better Thermal stability     * Better Oxidation stability     * Longer service life

 * Increased component protection     * Better resistance to the formation of Laquers and Sludge

Group III - The start of Synthetic Base Oils

The API defines the difference between Group II and III base oils only in terms of V.I., viscosity index. Base oils with conventional V.I. (80-119) are Group II and base oils with an "unconventional" V.I. (120+) are Group III. Group III base oils are also called unconventional base oils (UCBO's) or very high V.I. (VHVI) base oils. Group II+ base oils have the same maximum V.I. as Group II (80-119), but have a higher minimum V.I. (110-119).

From a process standpoint, Group III oils are made by the same process as Group II oils, but the V.I. is increased by increasing the temperature of the hydrocracker. The product V.I. can also be increased by increasing the V.I. of the feedstock. Which is done by selecting the appropriate crude.

Group IV - PAO Base Oils

"The word "synthetic" in the lube industry hase traditionaly been synonymous with PAO, poly-alpha-olefins, which are made from small molecules. The first commercial process for making PAO was pioneered by Gulf Oil in 1951.

Since then, the demand for PAO has grown and some base oil manufactures began using higher V.I. feedstocks to make mineral oils with V.I.'s that matched the PAO's. These new Group III oils were not manufactured from small molecules like traditional synthetics but they bridged the performance gap at a lower cost. Some lubricant manufactures began replacing PAO's with Group III base oils in their "synthetic" engine oils. This created a controversy in the lubricants industry because some believed that PAO's were the only true synthetics.

The National Advertising Department of the Better Business Bureau ruled that Group III base oils can be considered "synthetic" because modern oils made using hydroisomerization technology have most of the same performance features of the early synthetics.

Group V - are used primarily in the creation of oil additives.

Esters and polyolesters are both common Group V base oils used in the formulation of oil additives.

Group V oils are generally not used as base oils themselves, but add beneficial properties to other base oils. 

Note that the additives referred to in the Group V description are not aftermarket type oil additives. The additives referred to are used in the chemical engineering and blending of motor oils and other lubricating oils by the specific oil company that produces the finished product.

Summary of Base Oils


Viscosity Index


    Sulphur in %




< 90%

       > 0.03%

  Conventional (Solvent refined)





  Hydroprocessing or Hydrocracking





  Requires severe Hydroprocessing





  PolyAlphaOlefins (PAO)





  All other basestocks not in Group I ‚II, III & IV including  other synthetics