Good post, BraveRichard. You'll notice most of the companies pushing 3k intervals are Iffy Boob type quick-lube joints, or McParts stores trying to sell more oil. I'm a firm believer that 3k intervals does a lot more hard to the environment than it does to save your engine. Cheap dino oils do need 3-5k intervals or they WILL break down and leave nice sludge and varnish deposits. A properly maintained engine should last just as long on dino OR synthetc. A poorly maintained engine will probably last a LOT longer when fed Group IV/V synthetics - tons more 'safety margin' of oil performance to work with.

For the record, the recommended soot limit for MB diesels is 2%, especially with hydraulic lifters. Mobil-1 can suspend up to 2% soot, Delvac up to about 4%. It would not be wise to exceed those limits, IMO.