There is an important point regarding warm-ups that hasn't been mentioned so far. Unlike gas engines, diesels don't warm up much at idle speeds below 1500 rpm. Below 1000 rpm, many diesels won't maintain operating temperatures, which is why idle is good for cool down but not so good for warm-up. I give mine a few seconds at 800 to let the oil pressure come up, and then increase to 1500 rpm for the warm up.
What I've heard regarding extended idle times is that diesels idle well. They can idle all day. However, idling at below operating temperature results in incomplete fuel combustion, which contaminates engine oil. The oil should be changed more frequently in diesels that idle a lot. I know my diesel doesn't come up to operating temperature when I chug around doing loader of pallet fork work at around 11 - 1200 rpm. I notice that there is more smoke on acceleration than when the engine is worked harder.
Other than oil circulation, the reason for a warm up is that parts change size and shape at as they warm. Parts are designed to fit to clearance when the engine is at operating temperature, not when it's cold. The parts in a cold engine just don't fit together that well. For example, a cold piston isn't even close to cylindrical. They taper toward the top, which is hotter than the skirt during operation. They also are oval, to compensate for the extra heat retained by the wrist pin. There also are sizable gaps between the ends of the piston rings at room temperatures.
