I have a few thoughts. First off there is a filter integrated into the MB power steering pumps($12). It should be changed but I am not sure what the recommended interval is. I have overhauled 3 pumps and 3 steering gears in the last two months. All of them had about 180k miles. From my observations there was a lot of junk that seemed to accumulate in the steering box. It took a lot of flushing to get it all out. Then after I refilled the system and started the car there was a blast of dark colored oil from the return lines. This was after a complete overhaul and cleanout of the system including new hydraulic return lines.
In conclusion, it seems to me that much of the crud accumulates in areas that dont see enough flow to keep the dirt suspended so adding a second filter probably will not do much. ALso a good oil change will probably accomplish the best one can do to get the system clean. And also the oil breaks down over time and turns dark regardless of how much filtering is done. So just replace the fluid and filter from time to time and it will probably outlive you.
Just my thoughts.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k