An oil is any substance that is in a viscous liquid state at ambient temperatures or slightly warmer, and is both hydrophobic and lipophilic.
However, we more normally use the term oil to describe crude oil that is used to power the industrial world.
The distribution of conventional oil resources between different sizes of field is increasingly well understood. Although there 70,000 oil fields in the world, approximately 25 fields account for one quarter of the global production of crude oil, 100 fields account for half of production and up to 500 fields account for two thirds of cumulative discoveries. [Sorrell et al]
S. Sorrell, J. Speirs, R. Bentley, A. Brandt, Richard Miller "Global Oil Depletion - An assessment of the evidence for a near-term peak in global oil production" UK Energy Research Centre, ISBN 1-903144-0-35
See also: American Petroleum Institute, Barrel, Barrel of Oil Equivalent, Base Oil, Black Oils, Blind Reference Oil, Candidate Oil, Compounded Oil, Degrease, Dispersant, Engine Oil, Foam Inhibitor, Fossil Fuel, Gear Oil, Grease, Lubricating Oil, Mineral Oil, Oil Detergent, Paraffin Oil, Paraffin Wax, Petrol, Petroleum, Reference Oil, Straight Oil, Synthetic Oil, Test Oil, Tonne of Oil Equivalent, Transformer Oil, Used Oil, Viscosity Index, Wax.