All oil leaked in resivoir then overheated
The pressure in cooling system pushes coolant threw to the oil pan or cylinder if there is a head cracked or head gasket issue or crack in the block, so I would see if there is a oil cooler mixing oil in the system that leaked in some external oil coolers have antifreeze hoses to them to help bring the temp up when cold or stabilize the temp or a trans cooler leaking in radiator. A exhaust leak in to the cooling system can turn the radiator fluid dark and leave oily stains in reservoir.
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