I pulled the head off today, and my mechanic friend was right. I have a hole in the head, visible through the hole where the exhaust elbow and the head meet. I did not have to remove the head to see this, but it had to be done anyway... could not see a hole while the head was mounted.
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My engine was fine before I serviced the elbow, but that was only because layers of carbon were keeping the hole shut. I cleaned them off... and immediately after I started to have oil mixed in with the water. So yea, I've found my problem.