Don, Mike is right... many systems don't have the efficiency to tolerate a switch to R-134. For this reason the EPA has allowed some of the properly designed propane/butane mixes to be allowed, providing the system valves have been changed and are properly labled, and that R-134 has been demonstrated to be inadequate for the task. Most of the mixes like Freeze-12 are actually more efficient than R12, and way more efficient than R-134. They mix properly with the lubricants in R-12 systems, and will not cause compressor failures if installed properly. One thing to remember when using these types of gasses is that it doesn't take as much refrigerant to charge the system - ie where you might have used 4 16 oz cans of R-12 to charge a system, you would only need 3 16 oz cans of Freeze-12, due to the difference in the liquid volume. It is my understanding that these propellants are standard equipment in European Mercedes-Benzes and have been used extensively in Australia as well. If you are worried about safety, just keep in mind that your average can of hair spray has almost as much propane in it as your car has refrigerant.
'74 280C (with R-12 alternate for 5 yrs now...)
'77 Jag XJ6L (same)
'77 280Z (same)