Could be a variety of things. I'd start with a compression test. Since it is oily, could be a valve related problem. Could be a leaking injector. The LR book says to replace injectors every 80K miles, though I did run mine until 125K and the engine was still running smooth with the old injectors. You can have your injectors "pop" tested at a diesel injection repair shop. Keep track from which cyl each comes from.
The glow plugs should be Ohm tested (cold) and the results should be similar, within 0.1 of each other. I only use mine in the winter, as needed, and pull the fuse that makes them work the rest of the year.