C13 w. DPF, Oil loss to the point of low pressure warning

I'm looking for insight into this situation: C13 runs for about 2 hours after topping off the oil, then the low oil pressure light comes on, oil is added and the cycle repeats. So far, the following items have been checked: 1) No noticeable oil in the fuel tank, 2) No noticeable oil in the coolant, 3) No external leaks anywhere, 4) Nothing unusual in the crankcase vent/filter, 5) Turbo has been swapped for another, made no difference. 6) No unusual smoke. 7) No evidence of fuel in the crankcase. I believe the oil is being very well atomized prior to burning, as in, it's entering thru one or all the injectors. My theory is that oil is at higher pressure than the fuel at some location where normally a gasket or seal would separate it from the fuel. Examples, fuel pump where it mounts to the engine, or sneaking past the tappet and getting to the plunger of an injector. I also suspected seals in the turbo, but the swap effectively eliminated that as a possibility. And... folks have reported that C13's like to burn oil if overfilled, but that's not what's happening here (because it continues to burn oil until well below normal fill level and until a warning light comes on). I hope that's enough info to generate some ideas, and better yet, I hope a few of you have seen this before and can speak to it from experience. I'm confused and lost because the oil pressure is generally 25-40psi, and I don't know of any places where the fuel pressure is less than that. That's what I will hunt for without good advice from you... Thanks!