Gotta stick my 2 cents in here, Been working in an automotive machine shop for over 20 years and have seen sludge with any oil no matter what the brand if it was neglected. I have seen clean engines with all brands of oil as well. I have personally used Castrol brands of oil since their introduction and have yet to have a single sludged engine. I do know if you add even a small amount of coolant to the mix you have instant sludge.... you will not even notice if you have a very small leak into the engine but your oil will be sludging. The very worst oils I have seen for sludging were the oils that were based on crudes that came out of the Gulf. Those oils had clays in suspension that when the oil was overheated, those clays would fall out of suspension. I have seen Exxon oils changed every 3,000 miles and when the intake was removed to replace it with a Edlebrock Performer the sludge level was even with the bottom of the stock intake.... I could not believe my eyes. I knew this guy, his truck was well maintained, I still feel like there was something else wrong but could find nothing.
TJ