Yeah that's pretty cold! I like when it gets down to where Celcius and Farenheit cross (around -38) When we were camping in Scouts you could run around in the snow in your long underwear and socks (no boots on) and the snow wouldn't melt on the socks. I've never ridden my bike in that cold.I've been known to bit colder than 4°f, at -10, no problem, tried to go riding once @ -25°f by the time I got started, the temp rose to -15
you do realize I am talking about using it in my Vee, not a car or truckYou'll have no issues with using any 5/40 oil in the winter.
I am well aware of all the things you speak, and more, I've even started a campfire under a log skidder to warm it upI am aware that your are wanting to run it in your bike, but the only winter/cold temperature comparison you are going to find with 0 or 5 weight oils, is those used in the auto/truck world. I'm thinking of things like pour points etc.
If you really want the bike to start consistently in -15 F then you should be looking at a supplementary heating system, we call them block heaters/circulating heaters. Then you need access to power when the bike is stopped outside. Short of that there are many other things that people do in a cold climate to try and get something to start in the morning: ie: take the battery in/out of the bike, put a light bulb under the oil pan, drain the oil when you shut the engine off and put back in the morning.
If you check and search some of the adventure sites I'm sure you get some good ideas on how to make it work.