Definition of "oil"

The dawn of the oil age was fairly recent. Although the stuff was used to waterproof boats in the Middle East 6,000 years ago, extracting it in earnest began only in 1859 after an oil strike in Pennsylvania. The first barrels of crude fetched $18 (around $450 at today’s prices). It was used to make kerosene, the main fuel for artificial lighting after overfishing led to a shortage of whale blubber.

2013 August 3, “Yesterday’s fuel”, in The Economist, volume 408, number 8847

Yet, in another way, I was unable to put Picasso's oils in the same class as Cezanne's, or even (which will no doubt shock many readers) as Renoir's.

1973, John Ulric Nef, Search for meaning: the autobiography of a nonconformist, page 89

Such a vehicle is made by taking any old barrel (usually an oil barrel, but the one selected for our sketch was one that once contained Valentine’s varnish) and through each head of the barrel an inch hole is bored, and an iron bar is driven through, leaving the ends projecting about eight inches.

1884, Trade News, “A one-wheel Nantucket vehicle”, in The Automotive Manufacturer‎, page 372

Before they went to see Glinda, however, they were taken to a room of the Castle, where Dorothy washed her face and combed her hair, and the Lion shook the dust out of his mane, and the Scarecrow patted himself into his best shape, and the Woodman polished his tin and oiled his joints.

1900 May 17, L[yman] Frank Baum, chapter 23, in The Wonderful Wizard of Oz, Chicago, Ill., New York, N.Y.: Geo[rge] M. Hill Co.

The face which emerged was not reassuring. […] . He was not a mongol but there was a deficiency of a sort there, and it was not made more pretty by a latter-day hair cut which involved eccentrically long elf-locks and oiled black curls.

1963, Margery Allingham, chapter 17, in The China Governess‎