If the stick on heater wattage is too much, all you will be doing it cooking the oil at the bottom of the pan and over time you'll build up a nice layer of burnt crap.
All I do is make sure you are running a winter synthetic like Duron-E 0w-40(I run this oil year round), use a winter front(myself I have one I made and it's against my intercooler), and plug the truck in.
Ya one of the guys that quad with us put one in on his last year but big investment....Not sure if they're all the same but he didn't even have to start his truck, just pushed the button and his window defrosted and heated the cabif u plan on having diesel trucks for awhile get a espar deisel heater installed ($2500) that includes the blower for cab heat!! awesome ....when u sell yr truck just take heater off and put on new truck. Its a bit of $$ to spend but never have to worry about cold starting and cab is nice and warm!
FYI,I think your block heater does not work til -12 or -15 if I remember right. Kipp scott told me that when i complained my Enclave heater was not working.
Excellent suggestions on getting A high quality TRUE synthetic in that duramax, a pan heater I think is really overkill unless your going to the high arctic....lol The Espar heaters work amazing also. And I have fished out numerous block heater cords for the Chevy diesel guys in the last week, and the duramax does not have a -18C thermostat in the cord. Unsure about the gas trucks, but then all gas Chevys must run a DEXOS1 oil....
However since the question was about being unsure whether to pluggin in or not, just do it....saves a longer warm up time, and just less wear and tear on the motor...JMHO :d
Not trying to start an argument... but IF there is a high quality TRUE synthetic there is no need in my opinion. I grew up with diesels (GM 6.2L N/A - 6.5L T) and anytime I would start those trucks up with just a block heater the oil pressure would shoot up immediately.
Now if a cheap conventional 15W-40 oil is used I can see using a pan heater 100%...again JMHO