If it was overheated, you may be dealing with a warped head. Or worse a crack. Is your fan cycling on and off with the ac off? If it is then check the thermostat (should be replaced after a boil over anyway), and flush out the coolant mix, and just fill it with clean water for testing. With a new thermostat see if the temp stays normal, it should never go above the middle range if the fan is ok, as the fan cools it back down. If you have oil in the water, then I think a pressure test may be in order - and you may need to pull the head to track down the source.