Well, since not everyone is a multi-brand owner like we are, I'll answer this here too...

I run 0W40 full synth year round(used to switch to 15W40 for the summer, but, I can run the oil changes out long enough with the 0W40 that it's almost always once a year,) a Group 31 battery, and if it's not running, it's plugged in if it's colder than about 5F if I'm at home, elsewhere, down to about -20, I'll leave it sit for a max of 4 hours without running, colder and I'll either leave it run, or fire it up every 2. The only fuel additive I normally run is 2 stroke, but, if I'm away from home and buying fuel from an unknown source, I have a bottle of this in the truck and I will add some on a fillup:
http://www.kleenflo.com/en/productpages/973.htm
And, for all out emergencies I carry a spare fuel filter and a jug of diesel 911. In the past 6 years, with 4 different diesel rigs of my own, plus work vehicles and I don't want to think how many miles, I've never, ever, once had a fuel issue.
Locally, we start getting seasonally blended fuel in September, and the fuel I am burning right now is P50, which has a cloud point around -45 and a pour point around -52 by itself, but, it is blended just like Jet A-1, with anti icing and gelling additives in it. It's actually almost the same product, just not filtered to aircraft specs. Yes, it is low BTU, and fuel mileage suffers, but, I'd much rather give up MPG than have to deal with a dead truck.
The only time something won't start is if I forget to plug it in. My 7.3 PSD was a trooper, I fired it up once after sitting for 2 days at -40 on the glows alone, it didn't even crank over much longer than normal, BUT, by the screaming from every hydraulic pump in the truck it sure did hate me.
My 6.9 and my Cummins both have the same fault, battery capacity, I run a single group 31 in both of them, down to about -25, the Cummins will fire up on it's own if I leave the grids working(I am short one solenoid, so it could work better) colder, and it will be just about running, and they'll kick in while it's cranking and kill the battery. The couple times I have sucessfully fired it up colder, I just unplugged the grids and cranked until it was running, no real drama there.
My 6.9, down to about -20 or so it would go on the glows alone without much complaining, colder, and the same thing, not enough battery, so, I'd just skip the glows and give it a sniff of ether, pretty much guaranteed it'd fire on the second revolution. The 6.9 is a yard dweller for the time being, so it doesn't even have a battery kept in it, let alone reason for me to put in a second one. Heck, I haven't even put fuel in it since April, and I just had it running for a few hours on Monday with no issues.
The Cummins, I've just been lazy, and it's really freaking cold, I just need to turn the battery sideways on the holder and stick a second beside it.
Plugged in, either one of them starts fine with a normal starting procedure.