Do you think the brown area in this picture is rust? Or faded factory rust proof?


P.S.:
Anyway, when you degrease the engine room for engine detailing, is the MB factory rust proof removed too?
If you have any information about compatibility of aftermarket rust proof with MB factory rust proof, let us know.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.