R-12 is extremely expensive now. I have used butane mixtures in my old cars for 10 years, with no problems, Dura-cool but many other brands. It is compatible with R-12 oil and works slightly better as a refrigerant. You can read many strange rants about it. The EPA opposed it for years, out of supposed safety concerns (isn't that NTSA's job?). They bizarrely decided the only legal path is to first convert to R-134A, then butane. Yes, hydrocarbons are flammable, but so is Freon and R-134A. Indeed, the later forms poisonous phosgene gas when it burns. They put mercaptin in the butane's so you would smell a leak, and you don't put in hardly any (can feels empty).

BTW, the EPA is outlawing R-134A because it is a potent green-house gas. Not sure what they say about the "ozone hole" now. The deformed frogs in Costa Rica turned out to be due to a bacteria (not uV radiation) and there have always been 3-legged ones around.