oil in coolant

My GF's '95 Escort is starting to get oil showing up in the coolant reserve tank, but NO coolant in the oil. It's a 1.9/5spd,156k mi. My first guess is bad head gasket, but before I pull the head, anybody got a better idea.

my grandfather used a coolant bottle that I filled with old motor oil, to top off his radiator.  ooops.  would your GF do such a thing...
 
someone once put steering fluid in my master brake cylinder.  that sends your pedal to the floor, quite scary.
 
anyways.  probably the head gasket, warped head?  has it overheated?

Had a brother who's tranny fluid and coolant were swapped when his radiator failed internally on his automatic GMC Jimmy in 83'. Tranny oil is cooled via routing it through the radiator in its own sealed environment.

My GF doesn't put anything in the car except gas.It's an Escort  and a woman drives it,of course it has overheated. It eats fan relays but she always trys to "make it home so it's easier for you to fix".
I pressure checked the system(no leaks),flushed and refilled. So far no more oil. I got no clue. Guess I'll just let her drive it and wait.

Probably the head gasket, I got a low mileage Neon that was doing the same thing.  Pulled the head and found corrosion pits on head surface from where the coolant was never exchanged on 10 year old car.  Pretty important to do so as it will become ery acidic.  Element had a good idea, if only the car was not a 5 speed.