With all the cars running around in Sigil, there'd need to be a source of gasoline. Where would people get their oil?
Quasielemental Plane of Minerals?
Elemental Plane of Earth?
Paraelemental Plane of Ooze? (Eww)
Shurrock?
Acheron?
Any/all of the lower planes?
Athas?
Zakhara?
A large prime world-spanning drilling operation?
Any or all of the above?
Of Lower planes in mind spring sea filled layers (for sea oil platforms): in Baator Stygia, in Abyss: Great Abyssal Ocean (Layer that connects all sea layers of abyss) and Demogorgon´s domain (wich is also sea layer).
It seems like having oil found so abundantly in the planes makes it too common. On Earth, people kill over the stuff, and it seems like that's too much of a plot-motivator to write off. So, I think putting oil reserves anywhere where a plane matches the typical petroleum connotations of "geology," "dirty," and/or "greed" is going too far.
Except for some crazy Russian scientists, almost everyone in the field today operates under the theory that petroleum is biogenic, that it is the compressed and heated remains of organisms. So, maybe it could be rarer if you say it doesn't exist in any economic quantity in the Inner Planes, since there just aren't enough plants and animals around (though the Inner Planes being sterilized and lacking in inhabitants is one of the reasons people don't like 'em, so maybe not).
My idea: Great quantities of hydrocarbons exist on Thuldanin, the second layer of Acheron. The petroleum has mostly been petrified into great masses of stony fuel. Its unique properties make Thuldanin charstone popular as solid fuel sources, though it tends to ruin entire cubes of iron ore.
I meant present but not nescesarily common, planes without at least some native oil reserves would have major problems.
Perhaps the fiends have come up with a way to convert mortal souls into fuel-grade oil.
I highly doubt they would use oil to power 'cars'- there are far more accessible and useful means of transport available.
Aside from obvious magics (which may have been shunned in the past; but are now once again present) there are things like elementals, paraelementals, energon, elementites, etc.
These would all be viable energy sources; which may or may not sit well with the elemental lords ('oil' war anyone?).
Steam locomotion using water streams and an elemental would easily be put together; and would likely be more controllable than combustion vehicles.
Its cool to THINK of having a car in Sigil; but chances are it would be unlikely- the majority of inhabitants could not afford a place to KEEP a vehicle in the new Sigil (ie, if Sigil HAS grown; its likely more like a older european city which has been modernized- there will be those that have a 'car', but the cost of housing it and the fact that most locations within the city arent really worth 'driving' to makes it unlikely to be a majority issue.)
As Sigil grows, I would guess that public transportation would be taken into action; very likely under the command of dabus bus/trolly/subway drivers that can take people throughout the city. This further diminishes the need for an personal mode of transport.
On other planes and in other cities and regions; theres a good chance personal transport would exist- but in DnD (which is what this is really) I cant say I approve of having cars driving around.... thats something humans of OUR world worked up to circumvent the face that we dont HAVE a vast degree of alternatives (or, we didnt think we did when we began using fossil fuels).
I could even see a short period of fossil fuel using having gone by; replaced by new and better forms of energy (just as is slowly being done at this point in our world).
How about... petrol coming from the Elemental Plane of Earth?
Elementals don´t like their stuff being stolen; anyone who wanted fuel would have to get past them. ´Sides, the dao becoming oil sheiks is something... yo´d like, isn´t it?
You'll likely not have big SUV/roadster cars on Sigil, but those boxy Mr. Bean cars would be there. And bikes! Lots and lots of bikes!
Probably all of the above plus smaller doposits on every plane with a solid ground so they can still get thier oil.