I recall buying 5/16" Goodyear Barricade fuel hose for my OM617, from lift pump to main fuel filter, and same hose most other locations, even the ends of the "cigar" hose. Standard size on U.S. cars too, except some large engines use 3/8" on suction side. The small injector return hoses are 1/8", and I used Viton (ebay), but don't know about W124 cars.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans