I had some symptoms indicative of a head gasket problem - slight loss of coolant over time, some pressure left in the expansion tank after the engine cooled.

Well, the oil test results are back in. 0% water, 0% coolant in the oil. Not a trace. Oil looks good to average on most tests for a diesel of the age and mileage we are talking about.

So I guess a pressure test to find the pinhole leak I must have somewhere is the next stage?