Anyone know if anti-freeze goes "bad", opened and/or factory sealed?

What type Anti-freeze?

Engine antifreeze concentrate doesn't, premixed does and it's anywhere from 1 to 5 years.

RV antifreeze, if kept in the original container has a shelf life of at least one year and I've read up to ten years.

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]

All I've seen are Propylene Glycol. I like your idea about adding it full strength. It may waste some but if it gives you peace of mind then it's worth it.