In the early 2000s I traveled thousands of miles in an old school bus fueled by waste vegetable oil (WVO). The system was very simple, the bus had an extra fuel tank with a heat exchanger to the engine coolant, a couple fuel filters, and some hand-operated valves in the engine compartment. You start on diesel, warm up and switch to WVO. Before shutting down you would switch back to diesel to purge the system. You also pack a box of extra filters because they'd need frequent replacement. It was a somewhat popular conversation in some circles. Old Mercedes Benz diesel sedans were a common choice.