I know the compressor should have R134 seals as well as all the O rings. The oils are not compatible, so the system needs to be thoroughly flushed. The condenser for R134 has almost twice the number of fins, so the R12 condenser will work but cooling will be reduced. The STV which regulates the flow of Freon to keep the evaporator from freezing up does not work with R134, so you will need to get one of those A/C evap thermal switches that will cycle the compressor. Otherwise you will have freezing issues (air will not be able to get thru the evap). I replaced all my R12 seals and compressor for dual compatibility but have kept R12. The change to the R134 condenser made a lot of difference in my R12 as far how quickly it gets cold and how much colder it is in real hot days.
