Strange – I went through the same dilema recently and went back to the ingredients. All screenwash seems to be ethanol / methanol / ipa based. In pure form it works to -113 celcius! At 50% diluted with water about -50. 30% about -20. 10% -6.
So being a tight ass I had a litre of ipa (cleaning disks etc) and even 200ml in a 2.5L tank of cheapo tesco concentrated stuff works down to at least -20 over the last weeks. You may notice that I started another thread about ice forming on the inside of cars but screenwash wont help **** all in those circumstancs.
As a stop gap methalyated spirits will do the same job but it’s pink. So guys wont buy / use it. But my car’s handbook says to use it in extreme cases.
In a pinch vodka, gin (40% proof wont freeze till about -30ish). All of the above will probably attract the “my seals will rot” brigade, but the ingredients of screenwash are pretty much the same thing – just diluted with some glycol added.
I stand ready to be shot down in stw flames and am about to die etc!
Edit – dont fill the tank up when the engine is hot / running. All alcohol is very flamable.