Well I'll answer it and no doubt there will be someone who will differ, correct, expand my answer.
Don't know what you're flying but take the 355F; speaking from memory here.
In the limitations section it specifies anti-icing additive is mandatory below +4 degs C. So for winter in the old UK go for fuel with FSII added. If it doesn't have it then you need to add the stuff yourself as refuelling takes place. The stuff being PRIST and it comes in an aerosol sprayed in the tank to mix with the fuel. I seem to recall that you may find it difficult to find fuel with FSII pre-mixed so carry some PRIST with you.