The media of using air springs

Air springs are designed for use with compressed air. Nitrogen is also acceptable. Air springs may be filled with water or water-glycol (automotive antifreeze) solutions. If water is to be used, rust inhibitors should be added to protect the end closures.
Two reasons for liquid filling an air spring are:

  1. To reduce the internal volume of air (and therefore, increase the natural frequency of the air spring) .
  2. To use a media which is incompressible. Accurate positioning would be one reason to do this.

Petroleum base fluids (most hydraulic oils fall into this category) are NOT RECOMMENDED.
Moderately lubricated air will not harm the bellows.