With all the talk of sludge; I'd be trying a couple of very short change intervals in the next week or so, say 3-400km, with some basic 10/40w oil, then whip off a rocker cover to see just how bad it is.
If the heads are heavily varnished or sludgey; they are the barometer for the rest of the engine. They cop all of the heat cycling, so if they are bad; you know what the rest looks like.
Ideally there should be a light brown coating on the alloy of the head casting, but when you wipe it; it should clean down to the alloy. if it is gummy or black with gooey residue or even flakey bits; I wouldn't NOT use an oil system flush straight up.
I would be really hesitant about using a flush on a heavily contaminated engine; you run the risk of dislodging large deposits, and potentially blocking a piston cooling jet or even the turbo feed.
I think engine sludge is a bit like cholesterol, take your time to gradually break it down with very short change intervals, then start using very good quality oil, and obviously ensure that what you are buying is actually going in YOUR engine.
It's pretty morally destitute of your mechanic to pinch your oil for his own use; and frankly, I'd be going elsewhere.