I always add
Prestone Windshield Melt additive to whatever is in the reservoir. It's so strong and smelly that I only use half a bottle and it calls for a full bottle per gallon.
edit: Linky no worky. Go to
www.prestone.com, click products/ice fighters/windshield melt.
I actually have Prestone Bug Wash in there now with no antifreeze agent, and the added Windsheild Melt has been fine with our temps in the teens.
-Dennis