There seems to be a wide range of experiences. I'm not sure if you will see anything at all on B5 or B20. I noticed no changes in either of my Benz's constitutions on two "preparatory" tanks of B20 so I went on to B100, which was my goal.

Being as how my vehicles were already 20+ years old, most of the rubber lines had already been changed. My mechanic found one soft and weepy rubber line connecting a fuel return line to the tank but it was probably and old one that had never been changed. On my 123 car the lines have now had mostly B100 in them for over a year and appear unchanged.

I think that the reason people have trouble with rubber lines is that their car has not previously had the appropriate preventive maintenance. That is, the lines are 20 years old. No wonder they fail!

The lines to check are:
1. Supply and return lines (short rubber jumpers) from the tank to the steel lines that run under the car (the metal lines are not attacked by biodiesel)
2. Rubber lines in the engine compartment to the fuel filters and to the IP and the return lines, including the little jumper lines that go from injector to injector and terminate in the main return line.

Also keep an eye on the rubber gasket that surrounds the fuel filler pipe. Consider replacing the rubber gasket in the filler cap.

On both my cars, the fuel filters were changed as part of "post-purchase" maintenance. I noticed a little black crud in the primary filter of the '85 and changed it after a few months. Its tank and lines were apparently already pretty clean. The '87 has been on B100 for less time and still uses the filters put on at purchase.

Some people do experience massive amounts of black crud, melting hoses, etc. I did not. YMMV.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970