My wagon's self-leveling suspension has been perfect for years. Would someone please tell me the correct way to keep it that way?

This is what I have so far:

Change hydropneumatic fluid (000 989 91 03 10)
Change hydropneumatic fluid filter (129 327 00 91)
Change hydropneumatic pump oil (000 989 91 03 10 A)

Would someone please tell me if I am pointed in the right preventative maintenance direction?

Thank you in advance.

__________________
Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230