The mineral lubricating oils are made from petroleum crude after fractional distillation and refining. The chemical combinations of hydrocarbon atoms are infinite. For example 25 carbon atoms could produce more than 36 million different hydrocarbons, each with a different molecular structure. No two crude oils are alike; even from the same oil field they vary appreciably in their characteristics, molecular structure, and chain. So it is very difficult to classify them. In general, however crude oils may be classified by the type of faction produced when they are refined. Basically there are three main types of hydrocarbons and based on these basic ingredients the properties of the lubricating oil differs. They are as follows:
Paraffin hydrocarbons
Paraffin are straight chains of carbon atoms each joined with two hydrogen atoms. However the carbon atoms at either end have three hydrogen atoms connected unless they are replaced by side chains. Paraffins are also stable saturated compounds without any double bonds between the carbon atoms. The general formula for saturated paraffin is C nH2n+2
Naphthalene hydrocarbons
These are also saturated hydrocarbons in which all four valances of carbon atoms are connected to hydrogen or adjoining carbon atom, but the carbon atoms in each molecule is closed ring or cycle.
Aromatic hydrocarbons
Aromatics are also ring or cyclic compounds but unlike the naphthalenes, they are unsaturated with double bond between some of the carbon atoms. The general formula is CnH 2n-6 .
In general, paraffinic lubricants have good natural resistance to oxidation, good thermal stability, good lubricating properties, high pour points, high flash points; low volatility and high viscosity index (90 to 115). When used in lubricants for internal combustion engines, however, they tend to form hard carbonaceous deposits of very close grained nature. But they are suitably blended with naphthenic oils to remove the above drawbacks.
Beside the above classification the following type of classifications are used for cylinder lubricating oils. However in these classifications too it is generally mentioned whether the oil is predominantly paraffinic or naphthenic so as to get an idea of their basic nature.
