I think at one time Pennzoil and Quaker State used at least some levels of Pennsylvania crude which due to its higher paraffin content was a superior lubricant as a group I oil. That does not mean sludge. While oils like Havoline used oil that was more napathenic which act more as a solvent but does not give quite as good wear protection. I'm paraphrasing things so I might be a little off here. Again, this has nothing to do with the sludge claim except that the average person might have misinterpreted this to mean wax/sludge/pennzoil.
Anyway, I've known of people who used Quaker State exclusively since the 60's or 70's and never had sludge problems. Also, the sludge years I think happened after Pennsylvania crude was not used in any quanity, not that it was ever an issue in the first place, but it was an engine emission control issue that affected all oils at the time especially 10W40.
But what I don't get about people who continue the sludge myth, on every bottle of PYB it states, "No leading conventional keeps engines cleaner based on sequence VG sludge test". Now why would they doubt this industry standard test claim and that Pennzoil could make the claim if it caused more sludge than any other oils? Not only that, it also says on every PYB bottle that it not only prevents sludge it actively cleans up to 40% of sludge on the first oil change.