Coolant Reported in Oil

[TominAK]

I have a 1995 F-350 with a Powerstroke. I have been having Blackstone Labs test my oil for several years. I usually don't drive my truck more than 5,000 miles in a year. It has 83K miles right now. The last few years Blackstone has been finding potassium in the oil, and they say it's from the coolant. There's no sign of emulsion when I drain. Has anyone been told this by a lab? Were you able to track down the location of the leak? I'm wondering if there is a place to start looking besides the head gaskets that has been a problem. I remember from high school auto shop that there was a chemical that would turn color if exhaust was getting into the coolant. Has anyone used this? Thanks in advance.