I agree with the Many suggestions to the oil dye, that stuff is awesome!
If you think or wondering if it's blow by; plug all the vents on the engine.
Install a fuel pressure gauge on the dipstick tube, which I would keep low, like, 2~4 psi and feed it, through PVC grommet, compressed air
whilst the engine runs at a high idle. Make it 2700 for whatever reason.
Then you'll be able to replicate a high blow-by situation at cruse speed and find that damn leak. If that's what it is.
Best of luck.