How much Contamination could the R134 have caused to the mineral based oil
in a week or two of running with it ?

The Crazy Irresponsible side of me would want to just REMOVE the R134
Apply Vacuum Overnight and refill with R12.

Or is that asking for an HVAC Train Wreck ?

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'84 300SD sold
124.128

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

If he is going back with R12, then yes, that should work. If he's sticking with R134a though, he's going to have to flush the system and refill with the correct oil. It's up to the OP.

The oil needs to be compatible with the refrigerant to circulate throughout the system. Mineral oil will not mix or flow with R134a. It will get circulated to the evaporator and sit there, never returning to the compressor. Next thing you know, the compressor grenades and you're back to square one. PAG oil mixes with HFC refrigerants like R134a and POE behaves similarly, this is why you're supposed to flush and refill the system when converting from a CFC to HFC refrigerant.

It wouldn't be as big of a deal with a compressor with a sump like the Nippondenso used in later cars, but with an R4 which already has oil circulation problems, you're asking for big trouble in a short time by not doing the job correctly.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]