+1 on proper winter tires. They'll turn some white knuckled stress into a what, was that black ice on the road...?
Regarding oil, my owner's manual (1987 911), shows 10w30 as good for about -10F to 65F, while 5w30 is for -25F to 32F. From personal experience living in areas that can get really cold (-40F), if you can't plug your vehicle in overnight (pretty much EVERY 911), don't let it sit for the whole night in zero range temps. It might not start again in the am. I'd consider trying to find one of those magnetic oil sump heaters to use if the temps go really low. It might not help the carbs to their job, but if it does fire up, your oil shouldn't be a solid block of molasses. Either that, or you'll have some nights when you don't get a lot of sleep.
As well, plan for the worst. ensure you have cold weather protection clothing, and maybe a really good sleeping bag or two. Hypothermia is a real concern and kinda sneaks up on you so you don't notice it until it's too late. Getting to WA alive is likely the prime goal. Same thing with CO gas with your heater going all day long. Yes, you plan on getting your rusty HEs welded up, but are you sure they're going to be 100%? Pick up an inexpensive battery powered CO detector from Home Depot or something. Cheap insurance.
But most of all, good luck on your adventure!
