Potion of oil
Characters that start the game with an oil lamp will have potions of oil already identified in the discoveries section.
The potion of oil has a surprisingly wide number of applications, but is primarily intended to be fuel for oil lamps. This is done by dipping the lamp into the potion of oil, which uses it up - if the lamp is lit and the potion is not, the potion will explode instead. Magic lamps and oil lamps behave the same when dipped in potions of oil.
Dipping other objects
Dipping a unicorn horn into a potion of oil gives the horn an "oily sheen", which identifies and uses up the potion, but has no further effects. Dipping anything into a lit potion of oil uses up the potion and will burn the dipped item(s) if possible.
You can light a potion of oil by applying it, and snuff it out by applying it again; this also auto-identifies the potion. A lit potion of oil can be thrown as an explosive projectile, dealing 4d4 fire damage to anything caught in the explosion.
Quaffing an unlit potion of oil only abuses your Wisdom and identifies the potion; quaffing a lit potion additionally deals 3d4 fire damage (1d4 if fire resistant). However, if you are polymorphed into a monster that likes fire (e.g., fiery monsters such as fire vortices and salamanders), you instead exercise wisdom and take no damage.
Oil can also be used to fix damaged iron weapons; dipping a rusty or corroded weapon into a non-cursed potion of oil removes one degree of damage.
Identifying a potion of oil is fairly easy, as it is the only potion that you can apply, and randomly testing potions using this method is generally safe - however, doing this with a potion owned by a shop will result in you being charged a Yendorian Fuel Tax along with the price for that potion.
The scroll of fire, spellbook of fireball and other fireproofed items can potentially be identified by dipping into a lit potion of oil, but this method is ultimately a waste of a potion (as well as likely wasting whatever you tested with it) and is not recommended.
A lit potion of oil can be used to cure sliming if thrown upward; quaffing a lit potion of oil does not cure sliming.
In NetHack 3.4.3, it was possible to indirectly identify the potion without actually applying it (and therefore avoiding payment in shops) by dropping everything else that could be applied, then hitting a. If the potion was a potion of oil, the player was asked what they wanted to apply; otherwise, they would be told they have nothing that can be applied. As of NetHack 3.6.0, the applying prompt always appears as long as the player has at least one potion in their inventory, regardless of whether it can be applied or not.
- Ahh, a refreshing drink.
- You drank a lit potion of oil, and you are polymorphed into a monster that likes fire.
- That was smooth!
- You drank a non-cursed potion of oil.
- This tastes like castor oil.
- You drank a cursed potion of oil.
In SLASH'EM, it is possible to disarm a fire trap with a non-cursed potion of water, similar to using a potion of oil on a squeaky board; this will also generate 1–4 potions of oil based on your Luck. Since oil can then be diluted back to water, the large number of fire traps in Gehennom offer an effective way of multiplying one's water potions.
In EvilHack, intelligent monsters can light potions of oil to throw at you.