Why did they all travel to 1885, 1955, 1985, and 2015. Why not some off numbers? Like 1953 or '56. Always with a five at the end…
1955 was when Doc Brown first had the idea of the Flux Capacitor. It was entered into the DeLorean as an example.
1985 was the year the movie came out.
2015 is 30 years in the future, a round number chosen randomly. 1885 is similarly 100 years past, a random choice of round numbers.
IIRC, Old Biff was shot. What was why he was stumbling out of the car.
This was left out but was explained in the “Making Of…”
In that cut bit, you actually see Old Biff ‘vanish’ after he gets out of the Delorian.
I suppose that, as a character in a movie, Doc could have done that, but in the real world, it’s not so simple. For one thing. what is the proper grade? I doubt that very many people even in the oil industry could, by themselves, refine engine-grade gas.