Oil was a valuable liquid substance.
History[]
The Oil Wars were fought over control of oil. Oil-Eater was a biological weapon designed to render oil ineffective. Worlds with natural reserves of oil included Earth, LV-187, and New Albion.
Behind the scenes[]
The novelization of Alien mentions that the refinery pulled by the USCSS Nostromo is processing crude oil in place of mineral ore as seen in the film.