Oil
Oil was a substance found on some words, sometimes mined as a source of energy. the Doctor complained that being dependent on "mineral slime" did not make sense. He suggested the energising of hydrogen as an alternative. (DW: Terror of the Zygons)
The Nestene Consciousness thrived on worlds with much Oil, as well as plastics, smoke and polution. (DW: Rose)
Humans drilled for Oil for millennia, first on their own world and later, after the Earth was depleted, on other planets. Greg Sutton was an expert in the field of Oil drilling.
During the late 20th century drilling of oil off the cost of Scotland started, by several companies including Hibernian Oil. Four Oil rigs were destroyed out of the cost of Scotland were destroyed by the Zygons. (DW: Terror of the Zygons)
By the 40th century Humans were still drilling for oil, despite the Earth being depleted. Companies like Oil Corp would use mecha Oil rigs to drill oil on distant worlds like Boukon. (DW: The Infinite Quest)
The doctor was once almost boiled in hot oil. (DW: The King's Demons)
