I know that when Pennzoil and Quaker State merged, they were using different filter companies. I think at the time Quaker State was Purolator, and Pennzoil was Fram. The separate suppliers continued several years. They were finally combined, with *I think* Fram getting the business. The same supplier was used for the next several years.
The shrinking filter size and consolidation of part numbers was an effort to get greater coverage with less inventory. That's a welcome concept to many installers, who make up the majority of PZL-QS filter purchasers.
Having said this, I've been retired four years, so I don't have direct knowledge on who makes them now.