It is illegal to use Propane or Butane in a mobile A/C system. Too flammable, in fact it is explosive. Just about all "replacements" are blended refrigerants. While they may work well, none of them can duplicate the original for total cooling capacity. What that means is, you do not get as much cooling as you would if you stuck with the refrigerant the system was designed for. You will notice on a really hot day it just doesn't seem to cool enough. Bottom line, if you can still get the refrigerant your system was designed for, even if costs a bit more, it is not worth converting. R-12 is still available on E-Bay and you need less than 3 lbs. Fix your system so it doesn't leak. Use a leak sealer if you have to. To convert a system properly is a big deal. You should drain all of the oil, flush, refill with different oil, replace the dryer, and then charge with refrigerant. The service ports are supposed to be changed and the system must be labeled so you know it was converted and how much refrigerant to put in.