What's your mileage?
What is the recently history of any abnormal overheating?
Did you see any oil in the coolant when you drained it?
Oil in the coolant before the ET failed?
Oil in the coolant is the best indicator of a failed head gasket.
Yes a failed gasket would allow oil into the radiator.
What does the oil look like on the dipstick?
How could a water pump allow oil into the coolant?