I'd suggest removing either the vacuum line from the turbo to the injection pump, or the rubber intake coupler between the turbo and intake manifold and taking a look inside. With my truck the turbo had started leaking oil and it was obvious once I looked in the vacuum line to the injection pump. It was also obvious looking in the intake after I removed the turbo, plenty of oil residue in both.
I also discovered I had a leaky injection pump, so it's hard to say for sure what was the exact culprit since I fixed both at the same time. Since you've already compression tested the engine I'd suggest looking at the turbo next. Make sure it spins freely, doesn't have any shaft play, and isn't puking oil into the intake and/or the injection pump.