"According to court documents, the settlement affects 1997-2004 Audi A4 models and 1998-2004 VW Passat models with 1.8-liter turbocharged engines.
"The engines were prone to oil sludge from coking deposits even when maintained according to the automaker's recommended maintenance intervals and oil-quality specifications, court documents show."
Read more: https://autoweek.com/article/car-news/vw-cover-maintenance-costs-sludge-damaged-engines#ixzz5jebDkzpK
Sludging isn't isolated to turbo motors, though. Toyota had problems with some normally aspirated V6's a few years back.
The VW issue was certainly related to the turbocharger overheating the oil, but they also had small oil sumps. The "fix" was to use a bigger oil filter that held more oil. Solid German engineering...