I would suggest sticking with the r-12. You will be better off in the end. Think of the expense and the technical hoops you are jumping through to save $70-80 worth of refrigerant. How much is the compressor? Condensor? Hoses? Seals? You need a full flush to do it right...

Also, expansion valve and dryer (which you probably need anyways even for r-12).

Those '70s MBs did not have the greatest A/C capacity as it was. Stick with the r-12 to get the most out of it.

'Alternative' refrigerants are an even worse idea.

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1986 300E 5-Speed 240k mi.