Where the heck do you add oil on a/c compressor?

[Thunder]

This is on my chevy truck. The old one wasn't working to well. I assumed when I disconnected the lines a bunch of oil would be in there. None was. I turn it upside down, no oil. I've never replaced one before. Any help appreciated. thanks

[Ernzo]

ON a Jeep York compressor, there is an oil fill/drain bolt located on the case. It looks like a single bolt with nothing attached in the middle of the case. Open it and look in or stick in a feeler.The lines have oil in them mixed with the R12/13.