Oil is a hazard in UnderMine. It may be found covering the floor, stored in breakable barrels, distributed as a result of an Oiled Glomp or Oiled Uberglomp, or from the Galoshes. Walking on oil slows the player and prevents them from jumping. Oil is flammable, burns longer, and does more fire damage than other fire-inflicting sources (such as imps, Gargoyle, Torches, fire on the ground and such).
Oil can be spilled over water and cover it.
If the player has the Galoshes, they will be unimpeded by oil and can move normally and jump in oil.
Oil that is located near a room exit can't be set on fire, so it is a way for finding a secret room if you have Galoshes.
Oil has also a common texture for all rooms.