Cat C12 has mechanics puzzled ..

It’s not condensation that is almost certain. It’s very typical for traces of coolant to show on the the top well before the oil in pan becomes milky. The blow by gives it away as well. Coolant will give you an almost steam type blow by after it warms up good. Pull the pan and put pressure on it is about all you can do. I have also seen the pressure test not show anything but the liner have a cold water leak at the bottom. That’s usually how liner o ring deterioration starts to show itself. Although I wouldn’t rule out the water pump, though a pressure test should show a leak there. You can also usually see a trail with a really bright flash light

 

Till you pressure test it with the pan off your doing no good. I see you drained the pan but did you pull it off? Your not gonna see it run out drain plug usually. It’s a very minute amount of coolant in the oil. Your not gonna miss it out of the radiator. That goop on top just doesn’t come from no where. Anytime I have found goop and or rust......I found an internal leak.