I am NOT a fan of what I call JUNK refrigerants, but in the case where R12 is illegal, the proper JUNK refrigerant might be a better alternative than 134.

Because I will not use them, I am not an authority on the various junk refrigerants, but faced with such a situation, I would find one that worked with the mineral oil already in the system, add an ounce or two of mineral oil, evacuate and charge with the alternative.

If you are looking at an alternative refrigerant, you need to be aware of some things. ALL JUNK refrigerants fall into one of two categories; flammables and blends.

The downside to flammables is obvious, what is not so obvious is that most of them will have a moisture content that cannot be removed because evacuation is done BEFORE the refrigerant is fed to the system.

The downside to blends, which should be your only alternative, is that if you get a leak, the components leak at different rates, upsetting the balance of ingredients. For this reason, when the system gets low, you must remove the remaining refrigerant, fix the leak, evacuate and recharge.