Potion of oil
The potion of oil is the only potion that can be lit for an extended period of time.
Quaffing this potion is not useful and in fact abuses your wisdom. It has other uses, however. One use is to untrap a squeaky board, similar to a can of grease.
The other uses involve lighting the oil by applying the potion. A lit potion of oil is dangerous when quaffed - it deals fire damage and abuses wisdom unless you are polymorphed into a monster that likes fire (note: this is not the same as intrinsic fire resistance). A lit potion of oil can be thrown at an enemy to cause some damage; no damage is incurred if unlit.
Oil can also be used to fix damaged weapons. By dipping a rusty or corroded weapon into a potion of oil, it loses one damage. To completely repair a thoroughly corroded weapon, you thus need three potions of oil: from thoroughly corroded to very corroded to corroded to uncorroded. Oil does not repair rotten or burnt weapons, nor armor.
A potion of oil can also be used to refill an empty oil lamp.
Identifying a potion of oil is easy: just try to apply every new potion you find. If the potion lights, it's oil, and autoidentifies.
