Check with your local building inspector on this.As a contractor I can not put regular car anti-freeze in a heating system but in our area it is legal for a homeowner to do this.I would strongly recommend that you have a back flow prevention device on the domestic water line that feeds the system[this is code in most places],and if you use car anti-freeze you will want one that has an atmospheric dump instead of just a double check system.Some one has suggested RV anti-freeze,never use RV anti-freeze in a heating system because it is an alcohol base and will evaporate out of the system in a very short time.Now with all that said I would still use the non-toxic anti-freeze for boiler systems rather than used car anti-freeze.Used anti-freeze could be contaminated with oil,heavy metals,and other chemicals and may also be acidic these are things that you do not want to add to a new heating system that you have spent I assume several thousand on.

wal-mart, RV antifreeze,

Yes there is a special anti freeze that you need to put in the system. don't use auto anti freeze. I think that $22 a gal seems a bit high but what if you system froze and cracked you solar panels. I would bite the bullet and get it in asap. Talk with you local distribute and they will recommend what type to use.