Oil is also important for lubrication. Don't underestimate the importance of petroleum products in ensuring things run smoothly!
It's also used in asphalt! Needed to pave roads!
Also, if you go to the fuel stations, there are also gasoline tanks underground. You can tell because if you look at the ground there's a sign saying "Petroleum spirit-Highly flammable"
So some cars used gas. Maybe some corvegas were equipped with gas engines?
The red rocket gas stations confused me right from the beginning.
There are only nuclear powered cars in the Capital Wasteland so it would be logical to assume that the Red Rockets were service stations for nuclear powered cars. The only gas operated vehicles are the motorcycles one can encounter here and there.
But then, as you already said, there is this strange petroleum sign on that underground storage.
So even if these tanks were meant for petroleum ... Where are the gas pumps ?
The Red Rockets dont have a single one.
So the logical assumption is that these storage tanks are not for gasoline but for nuclear fuel rods.
And if they are petroleum tanks and the Red Rockets offered service for both type of cars , combustion and nuclear..... where is the service part for the nuclear powered cars ?
I take it that the nuclear powered cars of Fallout 3 were directly inspired by the suggested Ford Nucleon from the 1950's. Ford had the idea to built what they called service stations which were like gas stations except that they dont offer gas but repair and maintenance services for their nuclear powered cars, replacement of depleted fuel rods or the entire reactor etc.
That huge "Rocket" seems to be more than just a brand sign. It looks like the cars were supposed to drive underneath it. Maybe there was a pulley in it which could lift a nuclear reactor or its fuel rods out of the car ?
But somehow this sign and the highly strange layout of the Red Rockets doesnt fit to each other.