Another urban myth that has been busted numerous of times. Yes, at one point (early 70s? When US haphazardly implemented closed-system emissions control and also compounded with the switching to unleaded gas, etc.), Pennzoil, just like most other dino oils available at that moment, suffered badly from sludging. But then again: most of these are due to user's negligence and not in as much as the oil itself. LOL if all these "ole tymers" still thinks this way as Group III/IV dino based motor oils are coming so close to that of their synthetic counterparts (specification-wise). Also: with electronic fuel injection/computer management control, ordinary motor oils can no go way beyond the OCI levels that we would never dreamt of a decade ago (forget about carburetted cars). sludging? No way! I just follow the 6K-kms OCI intervals using severe hydro-cracked Group-III oils and it's been running like a champ.