Back when Quaker State was produced from Pennsylvania crude via clay and solvent extraction, it did make an inferior motor oil. Now, though, all motor oil is engineered to meet a sufficient level of performance regardless of the type of crude oil it's derived from.Back in the 90's and even into the 00's you could tell a Quaker State engine just by the burned smell, as soon as you took a valve cover off. Clog up an engine indeed, horrible stuff. Saw it first hand myself.
It was a legit concern back in the day. I've never run Quaker State myself because of it, even though I readily admit it has nothing to do with the oils of today and the myth needs to die, along with adding ATF to diesel to clean things up.