You do want to do UOA and you do want to stay under soot loading of 2% (though some oils have dispersant packages that allow up to 4%).

I found back in 2003ish, via UOA, that 7k was about when I'd hit 2% in a 617.952 well tuned without any increased boost, turned up ALDA, etc., and mostly smooth highway driving. The car averaged 28-30 MPG consistently.

We run 5w-40 in all our diesels. Can't go wrong with it. If nothing else it helps cold startup, and even wen it is warm, the oil will flow around a bit quicker.

We can go over 10k without hitting 2% (have to check the UOAs, I think we are actually under 1% at that point) on our 96 E300D.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)