Hmmm...I don't see a 1980 W126 anywhere, and no 500-engined cars before 1984. In any case, you will be able to find instructions for checking and adding oil during a compressor r+r, as well as replacement procedures here:
http://mb.braingears.com/
Essentially, you drain the oil out of both compressors. Measure the oil taken out of the old compressor, and put a like amount in the replacement. There are some instructions on adding more oil if you don't get a minimum volume out of the old one. Also, there are different compressors listed, so make sure you're looking at the correct procedure.
As for MB world, I kinda dropped that board when I found this one. The tech section is way better here, and I was looking for a place to learn.