Dan,
I was pretty sure when I bought my 95 E320 that it had never been done (very black, no bottom of the reservoir in sight). Given all the BITOG discussion about gentle engine sludge cleanup, I decided to take an analogous approach with the PS system. I bought one of the inexpensive siphon pumps (squeeze the bulb to suck up enough fluid to prime it), two quarts of Advance Auto store brand PSF, and one quart of M-B's OE fluid.
At home in the evening I pulled the cover, used a pair of needle-nose to remove the filter, and then siphoned the reservoir empty. Put the filter back in place, filled to the mark, started the engine to let the pump circulate, stopped and topped back up to the mark.
After the run to work and back the next day, I had black fluid again. By the time I was through my 1st quart of AA fluid, it was taking two or three days of driving for the fluid to become opaque. At the end of the second quart of store-brand fluid I could still see the bottom of the reservoir through darkened fluid that had run for two weeks. Started in on the M-B fluid. When I reached the point of only having enough for one more reservoir change-out in that quart, I replaced the filter with a new one, changed the gasket on the reservoir, refilled, and declared the cleanup complete.
That was about 5 months ago. A fluid level check before I took a trip this week showed clean fluid, only slight darkening, so I think the process scrubbed the gunk out of the system pretty well.
Cost a bit of calendar time, but not much working time. From here on out I figure I'll just remove-and-replace my way through one quart of PSF a year (mostly so I don't have a partly-full bottle to keep up with), and change the filter every other year.
When I price a replacement pump (ack), or a replacement steering gearbox (double ack), it seems like a reasonable cost and work trade-off.
FWIW