I've done a 50 mile round trip this evening outside temp gauge showed -3 degrees the whole time. The washer jets didn't unfreeze the whole way. Infact I still had ice on my bonnet when I got back and I was doing some decent speed.
I'm putting pure concentrate in when I fill it up.
my rear washer jet hasn't worked for over a week in this weather, the fronts sometimes work, sometimes don't. usually find if I park in the sun during day they defrost but never work in a morning. joys of a black car.
I don't use mine much (car, i mean) so I've still got a fair bit of summer grade screen wash in it. time to get myself down to shell and buy some of their good stuff I think.
Check the temps on the ready mixed stuff. The pink one from halfords goes lower than the blue. And when mixing concentrate, the instructions normally guide you on a ratio for cold temps. I haven't seen any ready mixed stuff in the UK that goes to really low temps so I bought a couple of cans in Germany. Aldi were selling some the other day but I didn't check the temps.