Hello,
The Audi 5 cylinder diesel from the 1980s may have been related to the GM 350 diesel :p
My friend was a VAG exec and his company car at that time was a 100CD diesel. It had all the GM engine's problems, there were so many modifications that even the VAG techs could not keep up. The car had three replacement engines in about 6 years of service and eventually wound up in a junkyard, cannibalized for body and trim pieces. Then VAG turbocharged the thing, it was a disaaster here and only a handful of demo units were ever brought over here, all usually overheating on a customer's test drive
Mercedes diesels have some very *heavy duty* features, like the *6 bolt* head fastening pattern over each cylinder bore and substantial main/rod bearing sizes plus a fully machined and hardened forged crankshaft. There probably are many more, but
AFAIK, the OM636,621,615,616,617 series of engines are milion mile engines with ease.
I have heard that the OM601 is just as good on longevity, but have no first hand experience with it or the OM602,603 as the trend was away from diesel passenger cars and only a handful were sold on Malaysia. The new CDIs seem to be gaining ground, but are very expensive here, the C270CDI costing about US$100,000
Just my two cents.