Ok, I do HVAC for a living, kinda familiar with refrigerants. First is the AC system intact? If so, and just low on charge, there are environmentally safe drop in replacements for R-12 that are compatible to the mineral oil in the compressor. On 134A plants, they use a synthetic oil that is NOT compatible with the mineral oil from the R-12 plant and to ensure it doesn't coagulate in a bearing the AC system should be thoroughly flushed prior to the changeover. Unless you are performing some major renovations, or have the proper tooling to do automotive HVAC, I'd recommend the drop in. Google R12 substitute as you probably won't find it in your average parts store.