If I remember rightly, it was the Dungeonomicon that pointed out that most D&D worlds have no oil deposits. Most such worlds have huge networks of caves, the Underdark, stretching across continents, holding vast underground seas, and home to a myriad of evil civilizations. There's no room for oil down there.
So what if all those caverns once held oil? What if some ancient god or wizard decided to open a hole in the world, far down in the deeps, and drain it all away?
And what if somebody went and turned the valve the other way?
The world changes forever, and probably not for the better. The surface kingdoms suddenly have access to a fuel source more powerfully combustible than wood, and undergo something of an industrial revolution, thanks to the invention of the petrol engine. Cities become nosier, more polluted, and more crowded and dangerous, thanks to the influx of refugees from the underdark. The countryside, always a haven for bandits and monsters, is now roamed by the less social underdark escapees, who prey on travellers not riding in the safety of the new armoured, petrol-powered coach services. The dwarf holds are a warzone, as they are the largest territory that is above the oil level while still being underground, making them a valuable prize for many displaced underdarkers.
The elves, fey, and other nature-aligned folks don't approve of the situation. Eco-terrorist druids direct crazed, polluted elementals (oilementals?) to cause even more havoc in civilized areas, and bemoan the invention of the chainsaw.
What will the PCs do? Normal dungeon crawling, but with new tools, and a good reason why every underground area is chock full of evil creatures? Perhaps get involved in the crime and vice of the underdarker slums? Guard against monster attack on the new highways? Or perhaps seek a way to drain away the cursed oil once again.
(incidentally, this would be a perfect excuse to break out the d20 Modern rules, possibly along with the d20 Past and d20 Apocalypse books)