A typical illegal oil refinery.
Hundreds (thousands?) of them were working two decades ago in some regions right in field.
Not much harder than a moonshine plant.
Allows to fuel low-octane engines, multi-fuel diesels, and stolen high-octane engines (significantly decreases the engine lifespan, so don't fuel with it your personal BMW).
It's simple. The petrol composition is from pentane (boiling point +36 C) to heptane (+98 C) and octane (+125 C).
Thus, by warming the oil to touch-hot temperature, you vaporize away all light fractions, by heating to boiling water point - distillate and collect the low-octane fractions, then by further boiling - low-quality machine oil and hudron.
To operate and service the plant, a family of shepherds is enough.
Another option for diesels is, say, rapeseed oil, but you should make the injector hole wider.