The general wisdom around here is to change the fluid in the self leveling suspension every three years or so. I personally think that's a bit overkill - there is no contamination source for the fluid, and it is not stressed by heat. The fluid is Bilstein hydraulic fluid. It's available from your dealer at absurd prices, but can be mail ordered for ~$10/liter. About four liters are required to perform a fluid change. Don't use anything else in the system - it may attack rubber seals or o-rings, resulting in an expensive rebuild.
There is a filter in the return at the reservoir under the hood. Remove the top to the reservoir and you'll see it.
I have a Yakima rack for my '87 wagon. They make a very nice system for attaching their cross bars to the wagon's roof rails. http://www.yakima.com
Don;t know what you mean by top teather mounting???