The reason for the question was that aviation fuel gets more viscosus the colder it gets, and so at low altitude it sprays nicely out of the injector, but as you go higher and it gets colder, the spray pattern slowly reduces until in the extreme condition it is just a jet rather than a fine mist. Then it is very hard to ignite
It may get more viscous, but not to the extent that you are going to have difficulties starting the engine. The engine driven fuel pumps on the types I am familiar with ( C20's, PT6's and T58's) all supply more than enough pressure to get a good spray pattern out of the nozzles, even at very low temps. I have operated C20 powered helicopters at temps of -40 and had good starts, as long as the battery was up to snuff....