Summary: QUALITY OF PRESSURIZED AIR

Here's a short summary of helpful comments:

many people run their rotary oil pumps without any trouble.
They are long-lived, robust, silent and require little maintenance.
In high-humidity settings, however, the same pumps may cause major 
headaches.
The problems range from pressure drifts with resulting instabilities to 
corroded pressure regulators.
In those environments switching to oil-free membrane compressors helps.
Special attention needs to be paid to getting rid of water vapors, 
nonetheless:
people use chilled in-line sheath tanks as water traps or refrigerated 
air dryers.
Some of the latter require quite costly regular exchanges of the 
airdrying silica cartridge. An economic alternative would be an acrylic 
cylinder filled with 'Drierite' plus a particle filter to stop Drierite 
dust.

Finally, where money can be burnt, a huge tank of dry, compressed N2 
promises best results!

Thanks to Guy Hermans, Jeff Haug, Matthias Haury, Eric van Buren and 
Nigel Rust !

Holm