A/C Compressor Oil

What kind,wt oil goes in the old factory A/C compressor??? Where on the unit do you check the level and re-fill if required????
I'm looking at an exploded view in a parts catalog and I sure don't see any logical place to fill the oil at.(nor could I find one on the unit itself!).
Must be one of those "techniques" one develops over time.
Thanks
Darius