So if you're dumping windshield washer fluid into your vehicle and think "Huh. That's odd. It usually takes the whole jug."?
Pay attention to that thought.
I checked the label on the thing I was pouring the windshield wiper fluid into. It was labelled "coolant".
Should you do this, if your next thought is "Yikes. That might matter."?
Pay attention to that thought.
I called TechnoHasAClueWhatCoolantIsFor.
"Hello, I love you" I said.
"I love you too" he said.
"I hope you still love me 30 seconds from now." Deep breath. "Ijustpouredwindshieldwasherfluidintothecoolantreservoir."
There was a much longer pause than I was comfortable with.
"Is the van still running?" he asked, finally, with great care.
"No, I turned it off before I opened the hood."
Relief tsunami'd, cell to cell.
So the bottom line is, yes, it matters, no, don't do it, and *phew* dodged that bullet. Someone helped me find the part where the hose from the reservoir connected to the radiator and we disconnected that, and thankfully, in this weather, the risk of an overheated engine is relatively low.