1997 Pontiac Grand Prix Coolant has milky goo around rad ca

Just had the oil changed and the mechanic noticed that there was a white chunky substance around the rad cap and said that it was oil in the coolant. They checked the oil that was changed out and found no trace of coolant in the oil. Engine runs fine and does not overheat but it does heat up slightly at idle for 5 minutes or more but fan kicks in and it cools down. I have a 180* T-Stat. Mechanic also stated that the hoses where soft and should be replaced along with a coolant flush. Could it be a blown head gasket or lower intake manifold gasket? And should I change the DEX-cool coolant for the green stuff?