Originally Posted by Diseasel300

If you change refrigerants, you need to flush the system and refill with PAG oil. 134a and 152a both require PAG oil. R12 uses mineral oil. The importance comes with how the oil circulates, R12 is miscible in mineral oil, so it is carried along with the gas charge. 134a and 152a are not miscible in mineral oil, so it will not carry it through the system. You run the risk of starving the compressor for oil, or oillogging the condenser or evaporator.

If you're sticking with R12, stick with R12. I did that originally in my SDL, but after about a month and a half of use, the expansion valve got stuck open and stopped cooling. You can't buy a new R12 rated expansion valve (different orifice size than R134a), so I converted the system at that point. When the conversion is done correctly (flushed, filled by weight, correct expansion valve) there's not much difference in performance on these later W126's. Early 126's and the W123's had a crappy condenser that was barely adequate for R12. The Gen II's have a small-tube double-circuit condenser that's much more efficient with the newer refrigerant. The SDL cools just as good if not better than most modern vehicles in my climate and I don't even have the windows tinted...