Quote from http://www.heco.net/freezone.html

"There are a number of products on the market that use flammable hydrocarbons (propane, butane, isobutane in many combinations). Here are a few that we know about: OZ-12, HC-12a, Duracool 12a, Enviro-safe, Red Tek, Maxi-Frig, ES-12a and Auto Cool. According the EPA SNAP list, these are illegal to use in mobile A/C systems."

Run away. You might as well run propane in your A/C system.

Your best bet: Have an A/C shop do a proper R-134a retrofit. This includes Evacuation, new Receiver-dryer, 134a desiccant, expansion valve, oil and any needed hoses. This can be done for $60-120 depending where you go.