I do not see this in cars that are driven frequently. If the car sits in a humid environment for a long period it will gather moisture in the fuel tank (unless it was completely full - not likely!) rust will form then flake off the surfaces (including the filler neck) and settle into the tank. I had those symptoms on my 124 project car. The fuel was crappy after sitting for a year and a half on empty in the fog belt.
I replaced four primary filters before rust stopped appearing in large amounts in the filter. I carry a spin-on filter just in case it gets plugged. I changed it once and thats all.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!