If you have a newer VW TDI or Mercedes CDI/Bluetec CDI, 20,000 miles can be achieved with oil analysis. But on older 617s, I would doubt you get close. But if you are going to try it, then I would suggest getting the oil analyzed every 5,000 miles by a reputable lab like:
Blackstone Labs
Polaris Labs
Oil Analyzers
AV Lubricants
An UOA report will tell you if you can go that far or not.
Link to
What is Oil Analysis?
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