Oil always looks dirty in our motors since there is normally a good quart or so in the oil cooler that isn't getting changed. The day I check my oil and it looks clean I'd worry, good diesel motor oil carries the soot to help save wear on the motor.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k