Might be a good idea to try a UOA ... Used Oil Analysis. Lots of
places offer this service but I usually use these guys:

http://www.blackstone-labs.com/

Signs of excessive sodium and/or potassium in the oil, along with
accelerated wear (usually copper, lead and/or iron, is a surefire sign
of a coolant leak and will be apparent through a UOA long before you
will get other, telltale signs of one.

They can also spot other problems such as high insolubles and/or high
fuel contamination levels which may suggest faulty fuel injectors, PCV
problems, etc ...

Costs about $20-35, depending on the service. Want the best package?
Pay $35 and ask for the Terry Dyson analysis. He's a 3rd party gyuy
who interprets Blackstone's results in much greater detail.

--- Bror Jace