Get a Blackstone labs analysis on that oil you ran for 20k, the wear metals will be so high they will want to come to your house and do the oil change themselves, just to spare the engine from excess suffering.
I ran oil 3,500 miles before, and it had real high soot and slightly elevated iron particle count, I don't even want to know what 20k does. Has nothing to do with the oil wearing out, it has to do with the additives going away, and the particles/filth in the oil reaching a critical point where filtration no longer proves effective. I now change my oil between 2,700 and 2,900 miles depending on how much freeway driving was done.
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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)