An old MB diesel mechanic was telling me a joke recently about oil changes. He was joking around saying "make sure you watch those guys changing your oil. They might lie to you and just say they changed it. You gotta watch them to make sure they're doing it". Brand new oil will be black almost immediately on these old diesels.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)