Analysis is the only thing that will tell you... Make sure you get the TBN or TAN (total base number, total alkalinity number) done so you can see how much additive is left, and also get the soot % and viscosity done to see if the oilhas taken too much on. I cant imagine it has though, in a well tuned, newer engine.
Ill be posting an analysis of delvac 1 in a normal drain interval of about 3 months, 4k miles in the upcoming months from out of my 83 300d... stay tuned

JMH

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)