Oil In Coolant.

Yes the most common cause is a head gasket and more likely what is going on with yours. Next would be a cracked head and then the dreaded cracked block. However, you should be getting water in your oil as well. What happens is when you shut down your engine and there is now oil pressure then the pressure from the coolant being hot will force coolant into the oil system.
How much film are you talking about? Did you use a flush product or just water? This could be left over residue from previous work?