The reason why you pull a vacuum is to evacuate the air and water vapor in the system before putting in refrigerant. Remember, the chemical that does the cooling is the refrigerant due to its physical properties....air and water vapor do not make good refrigerants.

Oil is usually added to the compressor, but it doesnt really matter much since the oil circulates through the entire system anyway.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)