I was going with 5w40, but other than low temp viscosity what advantages would the 0w40 have?
Not much, unless you're hitting temperatures where you're exceeding the limits of 5w-XX cold cranking.
Where can you buy it? I have yet to see it anywhere.
Get this, with respect to the 0w-40. PU and PUP have been just about impossible to get up here, in any viscosity, for forever and a day. Yet, I can get the 5w-40 and 0w-40 Euro from at least two locations, provided I'm willing to buy an EcoBox.