Growing up in the 50's & 60's I heard that rumour a lot about Pennzoil & Quaker State from a lot of old timers, some of them had been mechanics years before. Although it wasn't about the oil foaming up in those oils, but rather that they were bad about forming sludge. And it was mostly about Quaker State having the problems.
From what I had understood in overhearing conversations back then was that there had been problems with those oils many years before, but the problems had been resolved and were now some of the best oils out there. But apparently the stigma lived on then, and apparently a remnant still lives on today.
Back then Daddy didn't believe the problem still existed and used QS in our '58 Chevy pickup and '61 Ford Falcon, and had no problems whatsoever. Our old pickup had 150k+ miles on it when he sold it, and he was very bad about not changing oil when he should have. The oil was always as black as Abe Lincoln's hat when he would finally get around to changing it, so that oil took a beating. I have used Pennzoil and QS off and on in my vehicles for many years since the 60's and never a problem.
So if that rumour did have some basis in fact, that was very long ago and is certainly not relevant today.