Hydrocarbon refrigerants are very, very compatable with R12 systems. There are many hot discussions under what circumstances they are considered legal/illegal and and equal amounts of discussion over flammability issues. I'm not advocating it's use, just making you aware. I find a 60% propane/40% iso-butane mixture to be ideal for leak testing with UV dye and light in an R12 system. The refrigerant can be evacuated at the end of testing and used in the BBQ.
Duracool and Envirosafe are two brands that approximate the percentages I like (I mix my own). FWIW. no flames
, please, as I'm not recommending it for use, just information.