Any old hard sludge or varnish will be softened up and washed loose by synthetic oil, and the hard sludge in particular will end up in the filter.
If you don't believe me, take an older engine run on dino oil with long oil change intervals and pull the valve cover. Note condition, then change to Mobil 1 and run for 1000 miles and pull valve cover again. Will look like a new installation, and guess what happened to all varnish, etc?
This is a known event, not just gossip.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!