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.