Do a cylinder leakdown test, it will help tell you where the oil may be getting in from. Compression test won't. If this problem started right after the engine was run out of oil, I would pull it and do a simple rebuild without bothering with all the diagnosic time. You know what caused the problem, so you know all the usual wear items are probably toast.

__________________
1998 E300 turbodiesel

America's Rights and Freedoms Are Not The Enemy!