This is northern Illinois, so temperatures of 0 to -5F are not
uncommon. How can I make soluble oil (20 or 30:1 ratio) a little bit
more freeze proof. Is there something that I can add to it, like
ethylene glycol, without the risk of ruining the pump, seals,
bearings, or anything else. My garage is attached, so it rarely gets
below freezing, but when that happens, I do not want to be caught.

At my old place, the shop got down to -30 inside. I had 50:50 ethelyne
glycol:water coolant. Never had a problem with broken pumps, seals, etc. I
did have a problem with working in those conditions VBG

I bet 10% would be plenty for you. FWIW, I was just saving drainage from car
radiators. Let it settle, skim off the clean stuff and add oil.

Karl