After reading a few threads on Benz hydraulic suspension systems, I realized the PO of my car had screwed up yet again.
When I was looking at the car I noticed that the SLS hydraulic fluid tank was empty, so I asked the feller (a mechanic) what was going on. He said he'd forgotten to fill it after the last time he worked on the car (about a year ago when it was parked) and handed me a couple liters of Castrol HSMO (Hydraulic System Mineral Oil) he had in stock - stuff used by Jaguar, Rolls & Bentley hydraulic suspensions. Not knowing any better and trusting him (HA!), I poured them in, bled the system with some help from a couple big guys who were standing around to act as ballast and everything seemed OK.
Of course, now I find out that HSMO is mineral based (like baby oil) and Z-HM is hydrocarbon. All they have in common is that they are green. Now I get to drain and flush the SLS system too! Yay!!!
Here's a link about Citroen hydraulic fluids. There's an interesting table comparing viscosity of various hydraulic fluids at different temperatures.
http://www.candokaraoke.com/tony.html