What would be the right term for windscreen washer fluid that e.g. gas station's personnel would use? C.f. traffic turn indicator would be called blinker. So I'm looking for an informal name used by common people.
I have done most of my driving in California and Washington. I refer to the liquid in question as either "windshield wiper fluid" or "wiper fluid". So the informal name (that I am familiar with) is "wiper fluid".
For windscreen washer fluid, the shortest might be washer fluid or, to remove any ambiguity, windshield washer fluid.
I'm assuming since you're asking about gas stations and not a petrol station and your use of windscreen, that your question is AmE related and not BrE related.