Might be worth looking on the barsleak or barseal site as there is lot of information there including photographs of contaminated coolant.
What exactly has been done to the car and the cooling system, has the rad been flushed or replaced and has it had any head gasket issues in the past which could have left a residue of oil in the system, a complete cooling system flush and a pressure test would be a good idea but first make sure the thermostat is ok, some of the repro ones on the market now are total crap.
Geoff
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