Some time ago, the following was posted on alt.auto.mercedes. Can anyone confirm or provide a link to a manufacturer's site that confirms the difference in lube oilhole between GM and MB compressors?

"A word of advice on buying a replacement compressor:
Any one you get that does NOT come from Mercedes will have the front bearing
cover installed UPSIDE DOWN. Look at the cover on the front behind the
clutch. It will have 4 bolts. Between 2 of the bolts will be an embossed X.
This is the position of the oil hole to lube the front bearing. The R4
compressor was originally designed for GM cars with the compressor mounted
on top of the engine. The X and the inlet/outlet port is UP. Mercedes used
the compressor but rotated it 180 degrees and mounted it under the engine.
This put the ports DOWN. Then they rotated the front cover so the X is UP.
Oil does NOT flow uphill. If you use an aftermarket compressor with the X
down, you will starve the front bearing of oil. It may last a week, a month,
6 months.... depends on the quality of the bearing they put in. "

Here is link to Google Groups:

http://groups.google.ca/group/alt.auto.mercedes/browse_thread/thread/8db733da96f6ae79/6f27b1d537b97996?q=mercedes+air+compressor+oil+upside+down&rnum=4&hl=en#6f27b1d537b97996