Oil from a leaking head gasket will eventually separate into black or brown spots floating on the coolant.

I suspect you used a commercial detergent flush and didn't get all of it out -- did you drain the block several times? If not, it's probably detergent. Detergent foam would indicate a need to drain and replace the coolant -- you do NOT want it to foam.

If it's oily, though, you have a problem. Note that there is usually some soluble oil in the coolant for the water pump seals. It's normal to see an occasional oil spot a couple mm in diamter floating around in the coolant tank. Soap, no...

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!