The rubber is indeed sensitive to almost anything and everything, especially biodiesel. I cover the filler gasket with a "diaper" made from rubber sheeting of the sort used for backyard ponds and streams. Scraps of this material may often be obtained free from installers. The diaper keeps spills off of the gasket so that it lasts longer.

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