I do 6000 mile changes, without oil testing. If you do oil testing, you can probably run until you get too much soot, on recent engines this will be 10,000 miles or more.

On 615/616/617 engines, I suspect 4-6 thou is more like it, they collect soot much worse.

Peter

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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!