Oil in the water

Just a thought.
Had a situation with my daughters Jeep Grand Cherokee 1996 318. Reddish brown oil kept getting into the overflow. I suspected an oil passage or head gasket leak. Turned out to be the trans cooler in the radiator had a pinhole leak. When trans had some pressure it would allow a small amount of trans fluid into the radiator and being lighter than water rose to the top and into the overflow. Just a thought.