Oil is any type of multipurpose liquid substances that share common properties; these properties typically include being combustible, slippery, viscous, and hydrophobic.
Throughout history, various types of oils can be used to fuel oil lamps, such as those used on the Teplan homeworld. Following the Teplan's contraction of "the blight", oil for these lamps was a precious commodity often used in trades. When Doctor Julian Bashir claimed to seek a cure for the blight in 2372, one of the Teplan inhabitants, Epran skeptically inquired what the cure would cost him, suggesting perhaps "a good coat" or "a tilo of oil." (DS9: "The Quickening")
Humanoid skin naturally excrete oils for their pores. In 2374, Dejaren, an HD25 Isomorphic Projection, expressed his disgust towards "organics" to B'Elanna Torres, describing to her how she was a "repulsive creature" that is "constantly shedding your skin and hair, leaving your oily sweat on everything you touch." Following the encounter, The Doctor joked, after having relieved Tom Paris from sickbay duties, that he wanted Paris to help him "sterilize every square millimeter of the sickbay," seeing as Paris had "no doubt" left his "oily residue on every hypospray, and your sloughed secretions on every console." (VOY: "Revulsion")