There is allot controversy regarding "R290" ie propane.
You have to use a filtered type due to a "tar" like residue in motor rated propane.
I am not saying use I am just mentioning what I did experimenting with it.
The comment about it having to be used in a system specifically designed for its use isn't really correct.
The "orifice" tube in the automotive AC is used to control the refrigerant flow through the system and kind of a filter as well.
The issue is allot of people comment about exploding if you use it. There is far more risk in a "explosion" in an accident with that great big fuel tank on your vehicle.
The other thing is 134a degenerates when exposed to high heat into phosphine gas - orderless and deadly. Ice labs when you see news of all the dead bodies found at one and you see the guys wearing "spacesuits" is because of that gas...
Heck there are case of this poisoning at Vet Hospitals due to a pet ingesting Zinc Phosphate. Used in pest poison. Contact with the stomach acid, etc. and it creates phosphine gas..
Back on the subject.
I've charge automotive systems with "propane" as a test refrigerant used to look for leaks.
Given that a new 30lb jug of 134a now is costing me 300.00; that is very expensive to use looking for leaks. The propane "test" gas costs about 10-20 cents..
I fix the leak; pull a vacuum and retest; no leaks then pull a vacuum again and then and only then recharge with the 134a..
Also R290 "propane" is very ozone friendly and allot more environmentally safe.
You don't have to construct a 100 million dollar plant to manufacture 13a and DuPont is the only one who hold the patent to it...
Also propane is actually used in many countries beside USA for this very use; automotive, etc.
Remember if you get that great AC repair done in Tijuana Mexico; the odds are the gas is propane...