FWIW, since R12 has become so expensive a lot of people started using
"BBQ12" for refrigerant in automotive AC systems.  Basically a mix of 60%
propane (R290) and 40% butane (R600a).  This is the same thing as Freeze 12,
Duracool and other alternatives to R12 and R134a.  Most people are shocked
and horrified to hear that someone would put propane in their AC system.  Do
a little research and find out about refrigerants and what they are.
You can use 100% propane, but the high side pressure gets a little too high.
The butane brings that down.
Propane works even better than R12 as a refrigerant.

Gary