The prefilter in my SD was dirty. I replaced it and it was dirty within a few miles. I suspected algae so I tried a tube of Diesel Doctor from the dealer. The parts guy was sceptical because he believed all Diesel fuel in this area is treated with biocide. Still, I park this car for weeks at a time sometimes with the fuel gauge light on.

I poured/squeezed in the tube into a practically empty tank and filled with 10 gallons. That half tank and a full tank later the prefilter looks almost new. I have to toss a coin to decide whether or not to replace the main filter which is also new. It runs better than fine so I'm tempted to leave it. I wasn't going to bother with the strainer but now I see the fuel line under the strainer is leaking so I guess I'll do it in one go. Ugh.

BTW, the SD is now my in-laws' but I get to work on it whenever I like

Sixto
91 300SE
87 300SDL
83 300SD