Wow. That thing looks a little corroded, but otherwise in decent shape.

You've only got air preload - that's the schrader valves on top of each fork leg.

It's constructed very much like the Rock Shox RS1. The Tech manual for the RS1 is here- http://mombat.org/Rock_Shox.htm -to give you an idea of what it's like to change the oil.

Since there are schrader valves on each fork cap, they should be easy to remove with a schrader valve tool.