One of the loggers in our area made a comment one day about warming up machinery in the cold.... once running (some preheat with a construction type heater) he would set one of his hydraulic functions (on a skidder blade for instance), to hit relief and leave it there for a bit.... the bypassing would heat up the oil faster than just idling of the machine. Seemed crazy but he made a decent living running equipment....
Some of these guys also just leave the machinery running 24/7. Fuel is cheaper than the warm up time.