In the summer, you can take a chance and run down to fumes. But in the winter, if you do get stuck or stranded, the engine will be your only source of heat. And you don't want to have to worry about conserving fuel and saving the planet right at that moment...you want to stay warm. You can run the engine indefinitely at idle to stay warm-or as long as you have gas. No harm will be done to the engine.
