Engine sludging is caused by failing to change the oil often enough based on driving habits and conditions.
My view of VW is vastly different from others. It's based on working for several VW dealers and seeing them day in and day out and not on a singular personal experience, internet complaints, and limited in scope surveys, etc. Personally, I think VW builds fine cars.
Note with many complaints there is a willingness to apply a standard one way but not the other. Example, and this applies to other makes also. Someone on this forum posed a benign question about a Recall on a GM car and a poster stated a Recall was a "sign of poor engineering and workmanship".
I then pose the question of does this poor engineering and workmanship standard apply to Hondas, Toyotas, et al. Based on the answer, apparently not.
For every single problem with a VW there will be a corresponding problem with (fill in the blank).