Isobutane/propane refrigerant will work very nicely. However, if you have a leak, air leaks in while the hydrocarbons leak out, and you can get an explosive mixture fairly easily.
I do not see how a charged system can get _any_ air in it. I have never observed low-side pressure that dropped below atmospheric pressure if it was at all working correctly. If you let it get extremely low on charge, perhaps. But it would have stopped cooling long before this point, and should have tripped the safety switch anyway. It's not like you wouldn't have had your warning. Water doesn't flow uphill, after all.
You also risk a large explosion if you have an accident that punctures the condenser.
A rapid decomp of an R134A-filled condensor is also quite flammable. The oil spray is the big component, but you add the extremely toxic combustion products of the somewhat-combustible (!) refrigerant to the party.
R134a isn't much more expensive, is inert, and conversion isn't that hard.
All somewhat true. It's the nasty side-effects that are the problem. (It doesn't work well, it doesn't last long, the required oils are _not_ inert, and correcting it afterwards is made considerably more difficult.) -- Jim
