In the third movie Marty finds the Delorean in the mine and travels from 1955 back to 1885 to rescue Doc Brown. However once they establish that petrol hasn't been invented and won't be for a long time yet, why doesn't it occur to them to simply go to the mine where Doc left the car and empty the tank from the Delorean to the car Marty brought back in time. They would both exist. I know it would then ruin the film but isn't it obvious and wouldn't Doc think of this seeing as its only been relatively months since he put the car in the mine?
Per hypnosifl's comment, Robert Zemeckis covered this specific point in a Q+A session in 1993 for the "Hill Valley Telegraph Issue #8"
"Interviewer: In 1885, when Marty tells Doc they're out of gas; why don't they just go to the Delgado Mine, dig up the DeLorean where Doc hid it and get the gas out of it?
Zemeckis Answer : "There are two logical answers to this one…
The car mechanic's answer: As anyone who has stored an automobile for a long period of time can tell you, you always drain all of the fluids out of the car before putting it into storage. Doc most certainly would have drained the gas out of the DeLorean if he was going to leave it hidden for 70 years. At the 1955 drive-in, Doc specifically says "I put gas in the tank" indicating that the DMC must have had an empty tank when they found it in the mine.
The time travel theory answer: Even if Doc had not drained the tank, he still would not have gone back into the mine for fear of creating a time paradox by accidentally damaging the DeLorean, the mine, or who knows what. After all, since Marty is now back in 1885, Doc's plan obviously worked, and worked perfectly. But what if Doc were to go back into the mine and accidentally cause a cave-in that causes even more damage to the DeLorean? What happens to the future of that DeLorean, when it's unearthed in 1955? And what might that do to Marty and the undamaged future DeLorean now in 1885? As an analogy, imagine a time traveler going back in time, finding himself as a child, and cutting off that child's hand with a meat cleaver. What happens to the adult time-traveler's hand? That would definitely risk a time paradox, and we know that Doc would never go out of his way to risk such a thing for fear of (in the worst case scenario) unraveling the fabric of the space-time continuum and destroying the entire universe.*
At the point that Marty went back to rescue Doc in 1885, the Delorean had already been sealed up in the mine. As part of putting a vehicle in storage for 70 years, Doc would no-doubt have drained all fluids including gasoline since those would degrade and gum up sitting for that long. We are shown that Doc had some fore-sight into the effects of storage, as he put the vehicle up on blocks to account for the tires rotting away in that time, so we can assume he was familiar with other aspects of long-term storage of vehicles.
While not shown on screen, Doc and Marty certainly had to give the engine fresh oil and gas as part of getting it ready to travel again after digging it out in 1955.