Boiler antifreeze (Propylene Glycol) is non toxic and actually breaks down quite easily when exposed to air.
Don't use auto antifreeze- It is toxic, It's not legal, If you have to drain the system - what will you do to prevent soil contamination - especially if you dump into a rural septic system?
Antifreeze doesn't reduce efficiency, it reduces heat flow. Some reports say by as much as 18%
Regardless -- please install a code approved domestic water back flow prevention system if on a well or public water system.