I do 4k, it depends on the engine. If you're running a 4cyl 616 (240) or 5cyl 617 (300) engine 4-5k is the highest you will want to run it due to high soot levels produced by those engines.

On the newer engines: 603 6-cyl, 602 5-cyl, 601 4-cyl and the more recent 606 6-cyl engines you can probably go 5-7k without worrying about it as they run cleaner and make less soot.

Shell makes a lower cost synthetic (Shell Rotella-T 5w40 Synthetic) that can be used also, more practical if changing the oil more often. I have decided to use that oil and change it every 4k instead of the Mobil 1 every 5k. Still saves a little $$, and its likely better for the engine. On the newer engines where you can run the oil 6-7k, the Mobil 1 might be a better choice as its a slightly better oil.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)