Pool / RV antifreeze. Are they (pool/RV AF that is) all Propylene Glycol BTW? If not, any preferences?
Also, any reason to use / not use concentrated anti-freeze (typically finding it in qts) instead of the standard diluted stuff typically found in gals? The logic in me says pour the concentrate in the line after blowing them and, *if* your plug doesn't quite hold or whatever let the water that gets in do the diluting.
[edit: spelling ability falls off 2 hrs past bedtime]