Excessive crankcase pressure?

[Moose]

Cummins sells a UV fluorescent dye you can add to the engine oil. (3376891) ~$22./each last time I bought some a couple months ago. It's one bottle per 10 gallons I believe so get two.

You can get a UV florescent flashlight on Amazon for around $10

These two things along with a clean, warm engine at high idle & low ambient light should tell you the truth. A small leak somewhere along that valve cover could be traveling around the perimeter of that gasket and blowing off as it sees fit.