2 things to note here:

First, I ALWAYS use the heater plugs on any cold start, whether it's 80 degrees out or 20 degrees out. Heater plugs are $8 each to replace; A 2.5 diesel starter is $250 + to replace and the multiple 10-second cranks are killing it.

Next, I'd recommend NGK Y208T heater plugs when the time comes to replace them. They're super-fast, compared to the originals. One of the best things I ever did for my 2.5.

Good luck!