coolant will go into the oil more than oil in the coolant bc the coolant is under higher pressure so if u take off the oil cap and its like foamy like milk shake like itll be a blown head gasket bc the coolant is leaking in. I dont wanna get ur hopes up, but i wouldn't just to jump conclusions as to say its a head gasket, thats kinda a worst case scenerio, it could very easily be a thermostat. thats what it sounds like to me persoanlly. If it is a headgasket, lets just say its gonna be more than $50! u gotta take the head off, possibly do machine work, O2 sensors might need to be replaced bc coolant kills O2 sensors, lots of stuff. I say just check under the oil cap first and look for foamy junk.