Book on my 1990 602 engine says 5k, presumably with the dino 15w-40 available back then. I've sent many samples to Blackstone over the years and they have said each time I could go 10k+ on the M1 or T6 synthetic based on the test results. Not as much soot load with the 60x engines and the synthetic base holds up longer. As long as Blackstone says my filtration is holding up OK, there isn't too much soot, the oil itself isn't too far out of spec and the wear metals look fine, I'm OK running long with synthetic.
For the 642 in the Jeep, MB recommends 12.5k for standard service on 229.51 spec oil, half that for heavy service (lots of towing, etc.). I'm towing about 20% of the time so I figure about 9-10k on it is about right. 229.51 oil isn't cheap (especially at 10qts) and is rarely on sale so I don't want to do it any earlier than I need to.
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Dale
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1990 300D 2.5 Turbo -155k
2000 E430 - 103k
2014 Jeep Grand Cherokee Ecodiesel 4x4 - 11k
2014 VW Passat TDI SE - 7k Bro's Diesel
2006 E320 CDI - 128k Pop's Benz
Pre-glow - A moment of silence in honor of Rudolph Diesel
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