4 cans sounds like too much. I have gotten decent AC with 1 can and plenty of charge w/ 2 cans. If you can no longer source R-12, I have use Duracool in my M-B cars and even my 2002. Works great and fine w/ your mineral oil. I don't fear the absurd web statements about "explosion hazard", perhaps since I did Phd research in combustion.

The AC compressor stopped engaging in my 1985 CA 300D. Culprit was the OVP relay which feeds the EGR box (behind right cabin kick panel). The rpm signal routes thru there, which the Klima relay requires. I just removed the OVP and EGR box and jumpered across its pins to pass the rpm signal on. Search for posts by others w/ photos.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans