The new compressor fitted "3 years ago" likely would have used PAG oil, the one I bought last year certainly did. The early cars like these also used O-ring seals for the manifold, not the funky gasket like the later cars.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Just to insure you used the gage set correctly, push in the Schrader valve with a small screwdriver and verify there is negligible pressure. Hold a rag and wear safety glasses. The gage sets can be tricky. Never screw in the port valves all the way or risk bending a Shraeder stem. You just screw in (open) until you see pressure on the gage. BTW, unscrewing (knob ccw) closes the port valve (unless I forgot).

Don't leave an R-134A system with no pressure for a long time since the PAG oil can absorb moisture from the air to form acids, which quickly corrodes aluminum. One gomer flamed Duracool all over the internet, claiming their HC refrigerant caused his evaporator to corrode thru. He had left his VW's system unpressurized for over a year before trying Duracool. I think the rep was correct to tell him he was FOS.

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