How often is it necessery to flush/replace the power steering fluid?
Power Steering Fluid Flush on 2005 Mercedes-Benz S500 4MATIC
I do not recall any recommendations for power steering fluid flush from Mercedes Benz, if your vehicle is equipped with ABC, that fluid should be flushed whenever the height of the vehicle is not maintained when it is parked. I do not believe ABC came on 4matic vehicles. Usually with MB vehicles, the power steering system needs repair before fluid would be flushed, so new fluid is added at that time.