Oil found in coolant reservoir, no coolant in oil. Head gasket? Cracked head?

[rafahy]

Hey, I recently bought an automatic 1991 325i off Craigslist and while I didn't see any when I was inspecting the car, the other day I noticed there was oil in the coolant reservoir.
However, there isn't any (visible) coolant in the oil, which is a good sign. Also, the car hasn't overheated at all since I've had it, stays around 1/4 the temp gauge
I did a compression test, and while the compression seemed on the low end, it was consistent across all cylinders. In the 105psi range, with the biggest difference being under 10psi. This was also with a closed throttle, cool engine, and not putting oil in the cylinders
I took out the coolant reservoir and cleaned it out as best I could, put fresh coolant in, and drove for around half an hour. Doesn't look like any more oil has gotten in the coolant either
Are there any other tests I can do to test the head gasket/if the head is cracked? Could it be something to do with the oil cooler?

Thanks for the help!

Tl; DR: Found oil in the coolant. No white smoke from the exhaust, it doesn't overheat, and no coolant in the oil. Compression was low, but consistent across all cylinders.

[e30davie]

How much oil we talking about? If the oil comes back, then a coolant system pressure test would give you a pretty good idea. Kit you plug on the top of radiator, pump it up to a bit higher than the radiator cap pressure and see if it holds. Should hold pressure indefinitely. Mine held for 30 mins before i removed it.

[rafahy]

A good amount, when I drained what was in the reservoir into a bowl, the entire top surface was oil. Not sure how thick the layer was.

Good idea on the pressure test, I'll get on that.

Thank you!

[jeffnhiscars]

The oil cooler isn't near the coolant so that's not it but don't rule out an internal crack in the radiator which would allow trans fluid to mix. Would look like oil if it were floating in the reservoir.

105psi is low but youre supposed to open the butterfly to run a compression test.

[nrubenstein]

Are you sure that it isn’t ATF in the radiator? The trans cooler in an automatic runs in there, so a failing radiator could manifest that way.