I saw an article yesterday that talked about a sludge problem with certain engines from a number of manufacturers, including VW, Chrysler, Toyota and Saab. The article said that these engines tend to form sludge no matter the driving conditions or maintenance. It appears that they may have hot spots where oil is held long enough to break down and plug passages, requiring replacement of the engine.
This is the strongest argument I have seen yet for using synthetic oil. I have never used it in my vehicles, but am now seriously thinking about it. Those of you using synthetic in your Explorers - what viscosity do you use and how often do you change?