ToME WikiKhymChanur: As suggested here in the forum by FubarObfusco, there could be special flasks of oil that give different effects (for a limited amount of time) when poured into a brass lantern. I thought I'd seen a reference to that somewhere here on the wiki, but I can't find it, so I'm copying some of it over here.
Oil of brightness (250 turns). Increases your light radius by one.
Oil of sunlight (50 turns). Causes your lantern to glow with magical light, doing damage to creatures which are afflicted by light.
Oil of Abramelin (25 turns). Increases your spell-power by one.
Oil of infravision (150 turns). Increases infravision range by 50%. (50% of zero is zero, yes.)
Oil of clarity (50 turns). Conveys resistance to confusion and fear.
As suggested by RedDragonfly, these oils could be different colored (i.e., "flask of crimson oil"), like potions.
JanPrevratil: To balance that out (and give the usual "colour game" a new terrain), some "negative" oils could also exist.
Oil of Aggravation. Self explanatory.
Oil of Fumes. Temporarily reducing the light radius, as well as INT and WIS (You refill your lantern with the black flask of oil. Strange fumes appear. You feel dizzy and your vision is impaired).
Oil of Stench. Temporary reduction in CHA, perhaps a weak poisoning effect.
Oil of Flammability. Makes you sensible to fire, perhaps with a chance of some of your scrolls burning each turn.
Oil of Dilution. Oil spoilt with water. Your lantern has 0 turns of oil since you had to empty it of the non-flammable contents. A low chance of the lantern breaking.
However, being totally unable to code, I am not sure whether these effects could be coded to appear upon the lantern being filled.
Vin: Oo, oo! This would have some wierd/nifty effects if you tossed them, I bet! Oils of flammability, though...what if you chucked it on a monster? Would it make the monster burn a few turns if you tossed a fire spell at it? That might be kinda neat, actually...
Patashu: Should there be an ultra-rare/artifact Oil of Eternity that makes a lantern burn forever? And, assuming oil types won't stack together, make the last special kind of oil also burn forever with it, or regular oil otherwise.
NeilStevens: Hey, that'd be neat, even if we don't have other oils, that Eternity one. IF we have ego lanterns of course, heh.
DarkGod: I like it too
LukeHaub: What would you do if you're lantern was really low on oil and you filled it from two different flasks? both effects at once?
IngeborgNorden: Luke, yes; I see using different oils as similar to drinking two different potions or reading two different scrolls in succession. (One game-abuse failsafe, though...Oil of Eternity combined with anything spells trouble; it might cause explosion damage and/or negate the eternal duration.)
BTW, I'd like to propose another oil type: Consecrated Oil (which grants Bless for 250 turns of burning).
LukeHaub: It should probably also do extra damage to evil creatures if thrown
Orvin: How about a thermal oil that provides a little resistance to cold and melts ice within the lantern's light radius?
PatashuPatashu: Another powerful oil could be Oil of Truth, 25 turns and identify objects lit by the lantern.
Orvin: Citronella oil that causes fear in nearby insectoid monsters and/or prevents their breeding? 
Sirrocco: alternate choices for Oil of Eternity: either cannot (a) be combined with ay other oil but adds Time resist or (b) as a semi-joke artifact for the Blue Flame, that turns an ordinary Brass Lantern into the Lantern of Eternity, with appropriate bonuses. (Or, better still, allow the Blue Flame to coalesce inside of normal Oils of Eternity, giving the Oil of Eternity of Eternity.)
Orvin: Here's another: Oil of the Void. When it burns, it consumes all oxygen in a certain radius, causing damage/death to living creatures in that radius, including the player, if he is not able to breathe without air.
Chlorinated oil - produces a poison cloud that follows the player around, damaging the player and non-resistant monsters. Light radius is not affected, and cloud radius is independent of it.
Oil of Filth - darkens the area that would normally be lit, increases the player's stealth, slows and poisons both the player and monsters in light radius.
Oil of Cloying Darkness - darkens area in light radius, and physically attacks monsters that wander into that area for a small amount of damage. Shamelessly ripped off from a story by Fritz Leiber.