I like RV antifreeze the best. Used oil would be cheapest, but sure makes a mess. I've used COC before also...it cleans w/water the following yr fairly easily. You can buy the RV stuff on sale sometimes & make OK buys. It's pretty safe, meant to reside in sinks & fresh water supply tanks of camping trailers. It's almost drinkable in diluted form.

I've also used windshield washer stuff....but it seems to evaporate & leave a "residue" (soap?). The RV stuff doesn't leave that residue.

I put RV in everything on the sprayer. Then I make sure that all valves are OPEN for the winter. Most importantly, that means ball valves. If you have a ball valve somewhere that did NOT get antifreeze in it and is in the CLOSED position, it will freeze & split. So-just to make sure, I leave them open. On some sprayer controllers, that means killing the power to the controller w/the valves open so they don't close when you shut it off.