Probably water pump lubricant. I've seen the same on my 280 SE, just a few tiny spots of dark oil floating in the filler neck, never change.

If you have a bad head gasket or cracked head, you will also get cold pressure in the cooling system, and quite a bit more oil than that!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!