There was a refined petroleum product called "petrol" available as far back as 1870 which was used (among other things) to kill lice nits and as a solvent. When internal combustion engines were being developed, this product, unchanged, was found to be an ideal fuel. Why couldn't Doc and Marty have bought some of that to refill the DeLorean?
This is all just speculation, but some points to consider:
- How many bottles of this “petrol” would be required to get to 88 mph?
- To what degree is this “petrol” compatible with combustion engines from 1985?
- There were no highways back then, where could you find a place to accelerate to the required speed? (If I remember correctly, flight was not an option.)
They were going to try alternative combustible liquids. The first they tried was alcohol. The strongest one the could buy from the local bartender if I remember correctly.
With that the intake manifold to blew out and then it would have taken about a month to repair it according to Doc. They didn't have the time for that since they were planning to leave before the next Monday to avoid Doc's death.
Probably if they didn't blow the intake manifold they would have tried also Petrol eventually.
Petrol wasn't invented until 1870 or 1871 and it seems likely it was a small potatoes British business. While I don't doubt that some of their product made it to the eastern U.S., it seems very unlikely it was well distributed and probably would not have made it to the Wild West, especially California.
Even if it was sold in San Francisco, what are the chances it would have made it to Hill Valley? As portrayed, the town had made it to the iron age, but that was about it.
Because that wouldn't make a good movie.
In reality they could have burned Kerosene that no doubt has a higher octane rating (150) than petrol of the time.
With a low octane rating, they wouldn't have gotten to 88mph because the knock sensors would have cut timing and put the car in limp mode.
Part of the issue was that they had a time limit. Doc (and then Marty) was going to be murdered in a week. They didn't have time to do much more than try what they had available to them locally.
Doc was a scientist. So therefore he should have known gasoline was a by product (useless at the time) of the process of making kerosene. So in theory should have been pretty easy for Doc to cook up. But then again the movie probably wouldn't have been as good.