People can be highly impressionable..that's my answer to this question. I admit I can sometimes be that way, depending on the subject matter.
How I came to Pennzoil avoidance was around 1990 when I bought a 5-year old Oldsmobile Calais V6 from a technician I worked with at an Olds dealership. He changed the oil religiously at 3K, and even though the car only had 20K miles on it, when I pulled the pan in an attempt to replace a weeping pan gasket, I found a thick layer of slime at the bottom of the pan. The bottom end looked ok, but the pan was less than pretty.
He had always used bulk Penzoil 10W-30 in that car, and only drove it when he was taking a long trip. (It sat in his driveway any other time, as he drove his truck to work).
I don't know what set of conditions conspired to form this slime in the pan. There were no head gasket issues, there was no coolant intrusion. It was perfectly brown, but thick and slimy. So, that's the day I swore off Pennzoil YB, after years of running it in the family's cars, lawn equipment, etc. After my dad saw the slime, he switched everything over to Castrol GTX (except the mower, which was too old to worry about). Neither of us want the stuff in any of our engines, even though I'm sure the forumlations have probably changed numerous times. Some things only take once to scare me off for a lifetime. I know other people have a very much opposite experience from mine, and to that I can only say "good for you".
Having said that...I happily use PP in both my cars, mainly because of all the information that this forum has provided me about it. Good UOAs, nice clean top end pictures, great sale prices, etc.