I remember an operator who had a 20L container where all the fuel drained on preflights went into. As he had a busy flying school it didn't take long to fill up and was used to run his car.
Back in the old days AVGAS used in car engines long term used to result in fouling up the valves. However if you were getting the fuel free it worked out cheaper to pay for the top overhaul than buy fuel from a garage.
Modern engines are much less tolerant and where as an older diesel wouldn't mind a small percentage of JETA1, or even used cooking oil in the tank, on a new motor designed to the latest emission standards this would likely result in some expensive damage.