I'd definitely hang on to it as long as possible. Diesels love highway cruising, and yours isn't fettered by all the emissions gear that make modern diesels only just able to run on our borderline American fuel. I'm not real familiar with the Audi engines of that era, but in VW and Mercedes diesels, half a million miles is not uncommon.
Plus, as I see you live in Oregon, it is legal to pump your own diesel
