Pennzoil is one of the top rated motor oils out there. I'd question any mechanic that thinks he can identify an oil by looking at it or tearing an engine apart. Sludge, FWIW, is not caused by an oil but water in the system that has overwhelmed the PCV system. Short trips, not running the engine at temp long enough, are the common factors in sludging an engine. Pennzoil gets my vote along with a quality filter like the Motorcraft or Purolator.