Not all AC compressors are created equal.  The majority of automotive AC compressors have lubricate within the freon that flows through the AC system and compressor and that is how those types of compressors stay lubricated.  York AC compressors on the other hand contain an oil reservoir within the case that lubricates the compressor.  Usually an oil reservoir in the compressor makes the compressor larger so this is good reason for using the freon lubricated systems, which allows for a smaller pump.  However when you convert an AC compressor from pumping freon to pumping air, the York style compressor is the more desirable compressor.  These compressors were commonly used on AMCs, Volvos, Fords, Porsches, International Harvesters, and earlier Oldsmobiles.  Acquiring a York compressor can be as easy as visiting your local junk yard.  I've seen them at swap meets and on Ebay as well and typically they go for around 50 bucks, sometime less.