Crude oils, from which lubricating mineral oils are obtained, contain thousands of compounds, some of which remain after the refining processes. Most commercial lubricants are mixtures where the composition is chosen for the proposed use. The major mineral oil components are paraffinic, naphthenic, and a smaller amount of aromatic compounds. The variations in composition directly affect lubricant performance.

Paraffinic oils are straight chain or branched aliphatic hydrocarbons belonging to the series with the general formula CnH2n+2. Paraffin's are saturated with respect to hydrogen. A typical paraffinic oil molecule with 25 carbon and 52 hydrogen atoms has a molecular weight of 352. Very high molecular weight paraffins are solid waxes, also dissolved in small amounts of mineral oils.

Naphthenic or alicyclic oils have the characteristics of naphthenes, which are saturated hydrocarbons of which the molecules contain at least one closed ring of carbon atoms. Base oils may contain a small amount of Aromatic hydrocarbons.

Aromatics are unsaturated molecules identified by one or more benzene rings or by chemical behavior similar to benzene. The concentration of the three major types of compounds in a given base oil is determined by chemical analysis. The results are given as %CP for paraffinics, %CN for naphthenics and %CA for aromatics. For example, a commercial base oil is 68%CP, 32%CN, and less than 1%CA. The amount of each type of compound in a mineral oil is calculated from equations given in ASTM D3238-90 which require values of molecular weight, refractive index and density.

Mineral base oils contain many minor components, such as sulfur, nitrogen, oxygen, metal compounds, and water. The type and quantity of these compounds depends upon the degree of refining. A bright stock is known to have better, but limited, boundary lubrication properties. Some of the minor components also provide natural oxidation inhibition for the base oils.

Paraffins are relatively uncreative and thus have better oxidation stability compared to naphthenes. Thus, they can be used longer at higher temperatures. For example, oil companies state that turbine oils are blended using the best paraffinic base stocks. However, because of the relative stability and other properties of paraffins, they have poor solvency for additives.

Aniline point is a measure of the solvent power of an oil. It is an indication of the solvent effect of oil on other organic compounds such as rubber, seals, gaskets, and electrical insulation. Aniline point is defined as the lowest temperature at which an oil is completely miscible, that is clear, with an equal volume of aniline. The lower the temperature, the greater the solvency and the more likely rubber and polymeric compounds will swell. A commercial "handy" oil has an aniline point of 90 degree C indicating it would cause more rubber swell than a paper machine oil with an aniline point of approximately 120 degree C.

Aniline point is measured by ASTM D 611 where a sample of oil is introduced into a vial containing an equal portion of aniline. The fluids are warmed and lightly mixed until they are completely soluble, then allowed to cool. The temperature at which a haze appears, indicating the two liquids are no longer soluble, is considered to be the aniline point. In addition, naphthenic oils dissolve greater quantities of gases than do paraffinics. Also, paraffins have slightly higher flash points than naphthenics. Because of their wax content, paraffins have higher pour points than naphthenics. For example, for two oils of the same viscosity at 40 degree C, the paraffinic would have a pour point of -18 degree C, and the naphthenic a pour point of - 43 degree C.

In general, paraffins have a higher viscosity index than naphthenics. For example, 100 and 70 VI respectively. The VI of turbine oils range from 96 for a low viscosity oil to 102 for a high viscosity oil. In contrast, a general utility oil has a low VI of 29 to 60. Paraffins generally have a lighter color than naphthenics.

Pressure viscosity coefficients for paraffinic and naphthenic oils are similar, even though the naphthenic base oils are more affected by pressure than the paraffinics. The range for naphthenics is 1.3 to 2.8 X 10-8 m2 N-1, and 1.4 to 3.4 X 10-8 m2 N-1 for the paraffinics depending upon viscosity.

Pure base oils are not suitable lubricants for industrial use because of their poor lubricating and stability properties. With oxidation and corrosion inhibitors they are suitable lubricants only under pure hydrodynamic lubrication. They are sold primarily to companies that blend their own additives into the base oils.