Again, what is "typical consumption"? Most manufacturers consider anything above 600mi/qt to be "normal". I consider 3k-5k miles per quart to be "normal" for my cars. I have heard of cars with 1-1.5k per quart actually using LESS oil after switching to synthetic, as the oil control rings free up inside the piston grooves. It can take a while but really can happen.
The engine you describe has massive MECHANICAL problems that simply cannot be blamed on sythetic oil, IMO. I'd be willing to bet a can of Diesel Purge that the engine described was NOT a Mercedes diesel, though. If so, what make/year/model was it? This would be extremely rare in Mercedes land, like way less than 1% of the time. The other 99% experience positive results. Just my $0.02...
Regards,