It was due to the high parafin content in PA oil, right? That’s what I always heard.
So says Wikipedia:
Pennsylvania grade crude oil is thermally stable and has a high viscosity index. It is generally free of asphalt and has only trace amounts of sulfur and nitrogen. It is also high in paraffin and other waxes making it highly desirable for refinement into petroleum lubricants such as motor oil. Its products are also valuable for use in certain hydraulic applications. By-products are commonly found in consumer goods such as cosmetics, and topical ointments.
But since they don’t site sources, I’m not sure this more than gossip, possible just stuff from commercials.