I won't knock synthetic oils, they can flow better in the cold, etc. But, when a cold diesel engine is first started, it rattle tat tat's and smokes. A different kind of oil will not change this life shortening experience for the engine, as it does not add any heat to the combustion chambers so it can run properly.
My 2670 will start when it's zero, just give the glow plugs a little time to work, and, rattle tat tat, smoke. Something it doesn't do on warmer days. Next winter it's going to see a for real block heater. Yes, it'll have synthetic oil too, but for different reasons than preheating the engine.