Lubricating oil is essentially composed of hydrocarbons, i.e. compounds containing exclusively carbon and hydrogen. The simplest hydrocarbon molecule is methane, CH4. This basic molecule is the main constituent of natural gas. It can be extended with the addition of more carbon and hydrogen atoms, usually forming into longer chains. Four carbon atoms in a chain forms butane, one of the main constituents of LPG. The atoms may also form side chains off the main chain, or form into ring structures such as the benzene ring.
Lubricating oils are just extensions of these basic hydrocarbon structures containing from 20 to 70 carbon atoms per molecule, often in an extremely complex arrangement of straight chains, side chains and five and six membered ring structures.

The lubricating oil molecules can be divided into three broad groupings:

Paraffinic: Predominantly straight chains, tend to be waxy, have a high pour point and good viscosity/temperature stability.

Naphthenic: Straight chains with a high proportion of five and to a lesser extent six memebered ring structures. Tend to have a low pour point. For this reason they are used as refrigeration oils. They are little used in engine oil.

Aromatic: Straight chains with six membered ring benzene structures.

In practice, no sharp distinction exists between these various grouping as many lubricating oil molecules are a combination, to varying degrees, of the different types of hydrocarbons.

The main point to bear in mind is that these molecules are extremely stable. Lubricating oil molecules never wear out – all that happens is that the additives in the oil wear out or deplete and need replacing.