I got long winded on my opinion of 134a conversions so I though it best to state my factual opinions of other refrigerants in a separate post.
As stated hydrocarbon refrigerants are illegal IN THE US. Last I heard they were being used quite a bit in Canada: Beware! Other refrigerants are blends. There is no approved way to recycle blends!!!! It you put these in your system they will have to be vented to do any future work on the system (which is probably illegal, it is at least stupid). The other alternative is to have it disposed of as hazardous waste. I pursued this once and found that a ten pound cylinder would cost me 2-300 dollars to dispose of using the only local hauler that would even touch it. I'm sure the owner of the vehicle, that had the unknown quatity (found during standard refrigerant indentification, before hooking our recycler), released the stuff to the atmosphere as he was pretty upset when we wouldn't work on the car. He wasn't upset at us. He was upset at the shop in Georgia that did it to him.
Bosch Master, ASE Master, L1
33 years MB technician