Seems like it depends on what type of compressor was originally on the vehicle. Some types of internal seals are incompatible with the R134a and/or the new refrigerant oil that it requires. The oil is a problem too, as the old stuff needs to be flushed from the system as thoroughly as possible. Also, some OEM compressors were marginal when running R12 and won't last long with the higher pressures that R134s requires. See the link for more info.