Danke! Das ist es, what I mentioned above...

7. Reverse Rotation
As mentioned earlier, Airborne pumps come in clockwise (CW) and counterclockwise (CC) flavors, designed to rotate in one and only one direction. The profile of the elliptical rotor bore is "note" symmetrical in the Airborne pump; also, the rotor slots are cut at an angle. (These design features may improve performance, but some experts feel they make the carbon rotor more susceptible to damage.) Attention to label instructions can eliminate incorrect rotation of pumps in normal use, but avoiding occasional engine "kickback" on startup (or shutdown) is not such an easy matter - yet if vane/slot clearances have opened up, one kickback may be all that's needed to jam a rotor and trash a pump. Pamco Industries' Thomas Zompolas (designer of a new standby vacuum system for Mooney's) feels this is a major reason why Airborne pumps used as standbys tent to last longer than the same pumps installed on engines. "That's why you often hear guys say 'but it was working fine when I shut down'" Zompolas maintains.

Ja, danke an M. Koepfer für die Recherche. Da steht aber auch:

The Edo rotor has eight slots and eight vanes, it can be turned in either direction. The Airborne design, by contrast, uses six slots and vanes each at a slight angle, giving the pump a preferred direction of rotation. Airborne units thus come in 'CW' (clockwise) and 'CC' (counterclockwise) models, and for long life, the proper sense must be observed on installation.

Hängt´s also noch davon ab welcher Hersteller verbaut ist?

Ausserdem war mir nicht klar, wie empfindlich die Teile auf Öl und Wasser "Verschmutzung" sind.

Hier noch ein Fund:

https://www.avweb.com/news/maint/182905-1.html