Hi, thanks for your answers.

Its not as if we go top speed every day over here. For me and my wife, driving on the highway is happening only when were going on holiday or some other event far away. Normally, we drive city and overland with top speeds of 120 kmh. My wife is quite a slow driver, she prefers to stay behind the trucks at 90kmh and never uses full throttle. Under these conditions, none of our cars had issues with carbon or anything like it in the engine. The cars are also equipped with Diesel catalysts in the exhaust, I would think those getting blocked would be an issue with bad burning fuel. Never had any problems with that. Bzt when you drive the cars really slow and dont rev over 2500 for a long time, the exhaust seems to get full of soot. When you rev it over 4500 then, you end up with a black wall behind you.
I experimented with much higher injection pressure (160 bar) and very early injection (up to 7 degrees earlier). The result was a better engine start without shaking in the summer, but a lot louder engine with less power and heat problems in the prechambers under full load.

The waste motor oil experiment might have been more successful with engine oil from a petrol car. The truck oil has a lot of soot in it and is used for 100.000km from change to change. But I think a prechamber engine might not be the optimal choice for this fuel. Direct injection will prevent any dirt from thr burning process staying in the engine.
I guess I will never know what the problem with myengine was. The car still exists, but I will drive it to Africa this autumn for holiday and leave it there.