JET A-50 means that it is a special low temperature fuel with a freeze point of -50 degrees. Normal JET A-1 has a freeze point of -47 degrees. Not much difference I know but I guess every extra degree counts in that part of the world.
As the FSII is used to prevent icing of fuel in the wing tanks during flight, and that at altitude it is very cold all the year round, it does not matter whether its summer or winter.