I rode for groceries when the temperature was 11*F. When I arrived at the grocery the key would turn the ignition on and off, but it wouldn't come out of the lock. A trip to O'Rielly's netted a little aerosal bottle of lock deicer. The bottle works by pushing its spout into the lock slot, or in this case, where the key entered the lock slot. 4 or 5 little squirts had the key working properly, and no problem since.
This comes up as a subject of discussion regularly where I live (anc ak) , particularly given the business i am in (school district locksmith). In general, the only thing WD40 has going for it is it is inexpensive, and more or less edible. It has low lubricity and requires frequent re-application. Graphite is a very good lock lubricant, required on our BEST removeable core cylinders. However, graphite does not displace moisture, and mixes poorly with the aerosol lubes the un-informed sometimes put into locks requireing graphite only. I like Triflo or LPS 1 for most conventional locks.
Ether starting fluids are my all time favorite cleaner and degreaser (not used around the schools) with alcohol as a more likely choice. You have to relubricate the lock after using any deicer. Going through the freeze thaw cycle causes most lock problems, you have less trouble @ (-) 40F than slightly above and below freezing. I lube my personal vehicle locks about every two weeks (triflo) during the beginning and end of winter. Of course our seasons are described as "almost winter", "winter", "not quite winter", "road building season".
I have ridden my TW to work down to (-)12F usually fires on the second kick.