what could be the problem?

What is your question?

no overheat, no drivability issues either. I just happened to check the coolant and the reservoir was full of chocolate sludge. My engine oil leaked in to my coolant somehow. I have drained the coolant and the coolant looked new other than about an inch thick of oil sludge floating on the top of it. How do I clean the coolant reservoir and the entire cooling system of all the sludge that is stuck to the walls of everything, ie. hoses, recovery tank, radiator. Am I going to have to replace it all or is there a way to clean it? how can I verify if the oil cooler is the problem and not a head gasket Thank you