A few manufacturers, including Toyota, have gotten burned by failing to specify the use of synthetic oil in an application where they should have done so, resulting in sludging. Toyota has rebuilt or replaced plenty of sludged-up engines.
VW diesels 2004 through 2006 (includes mine) are sensitive to oil viscosity. As long as you use a good diesel-rated 40-weight oil, they're fine. Use a 30-weight oil, doesn't seem to matter whether it's synthetic or not, or diesel-rated or not, or even meeting VW's own specifications or not, and they chew through camshafts and lifters. It has happened to lots of people. The newer ones have finally switched to roller followers, and no longer have this issue.
