Having driven over the road in a diesel truck since 1970 there have been many changes in the way diesel engines work. In the old days with inefficient mechanical fuel pumps and injectors the issues were hard to start, too much fuel washing the liners down when low idling, low oil pressure at low idle. Too afraid to shut the truck off for fear it wouldn't start I never shut it off until home again. Everyone put a hand throttle cable in so you could set the high idle up to 1200 rpm which solved the low idle problems and kept the engine warm and toasty. We always let the engine idle down 3 minutes before shutdown.
When Electronic engines came out idling was no longer an issue as the computer measured the proper amount of fuel at idle so no detrimental effects result from idling except the engine would cool down too much to keep warm in winter. By the time the new engines came out all the states had enacted 5 minute idling times for trucks so we had to stop but not for any equipment issues.
Diesel fuel (number 2 fuel oil) is not flammable. You can put a gallon of diesel fuel in an open bucket and you can't light it on fire without a wick. If you inject it as vapor into a closed container and put in an ignitor it may explode.
