Depends upon what is in there. If you are just converting a working system, 2-4 oz of PAG oil is all you need, as the mineral oil will continue to work fine. It's not dissolved in the freon, but it will end up in the compressor and do it's job. The PAG oil will circulate with the freon and lubricate anything else.

If you have the system apart (as in replacing a compressor) the best thing to do is flush very well and put ester oil in. It works better than the PAG in a clean system, or so I've been told.

The real problem is that you have crud flying around in there from a bad compressor, and it will eat a new one in short order. My friend the german mechanic always puts a suction side filter in the suction hose when he replaces a compressor. It helps, but he still has trouble with rebuilt compressors. They usually fail -- only the ones done in Japan seem to last.

Best bet is a new compressor (cheaper than three rebuilts!) and a very complete flush. Better yet is to replace everything -- condensor, lines, and evaporator. Some suppliers won't honor the warrenty if you don't on new compressors. Needless to say, this raises the cost considerably.....

You shouldn't be able to fry the compressor in a current system from loosing the freon -- the low pressure switch should keep it from running when the charge gets low.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!