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"
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.
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