turbos do puff black smoke on acceleration ,oil leak on gear box is often fuel pressure regulator on drivers side rear of head ,it leaks back running own over gear box worth checking
Yes, mine has the oil leak. Probably not too severe in my case as it doesn't leave drips where it is parked, but explains why the rear of that side of the engine, the various wiring looms and the starter are kept moist. It didn't seem very diesel-like to me, as a good deal of it was black and sticky. But evaporation and road dirt would account for that over time I suppose.
How bad does it have to be before it's worth dismantling?
If you are talking about the High Fuel Pressure regualtor, if it is leaking it deffo worth changing, whilst you may think its just a little oil leak. You will also loose a bit of power and find it a bit sluggish on first gear.
If on a ninety Im led to beleive it is a lot easier than a disco as it has very limited room to work on. If you have big hands and fat fingers you ,ight struggle.
I did mine on a disco a few years back and found the bit that fails is the rubber o ring and I think there is a replacement for it now, rather than buying the whole unit.