I used to use Pennzoil motor oil and FRAM oil filters all the time. I stopped using the FRAM oil filters when two failed on two different cars, but that is another story. I stopped using Pennzoil when a remarkable number of people who seemed to know something about motor oil told me that Pennzoil (and Quaker State) cause sludge in engines. I was told this by mechanics, a couple of race car drivers, people at new car dealerships, etc.
But if you look at the VOAs and UOAs that people have done and put the results up at this site, you will see that Pennzoil is one of the top performing conventional motor oils. As far as I can see, probably the three best conventional motor oils based on VOAs and UOAs that people have done are Chevron, Pennzoil, and Castrol.
I think what happens is that mechanics see a lot of sludged up engines in cars and trucks where Pennzoil and Quaker State motor oil was used, just because Pennzoil and Quaker State are so popular. If a motor oil is a very popular brand, you are bound to see some car and truck engines where that brand was used and the engine is sludged up. Why is the engine sludged up? Probably because the owner neglected oil changes, or let the engine get too low on oil, or put some sort of useless oil supplement into the oil. Some of these oil supplements and engine treatments are less then useless.
The mechanic sees all the car and truck engines with sludged up engines, and a lot of those engines had Pennzoil or Quaker State motor oil in them, and he adds two plus two and gets five. He thinks it was the brand of motor oil and actually it was bad maintenance.
The mechanic dos not notice the few cars and trucks that used less popular brands of motor oil and also have sludged up engines.
A lot of mechanics I know use Valvoline motor oil in their own cars and trucks. Why? Probably advertising. You know-more mechanics supposedly use Valvoline then any other brand. But conventional Valvoline motor oil has not looked as good as Pennzoil in the VOAs and UOAs that I have seen at this web site.
Actual testing (VOAs and UOAs) is going to show us the way to the best motor oils. You don't want to depend on word of mouth, what grandfather used, or advertising. And remember, even once we know what a good brand of motor oil is, that may change over time. What is a good brand today may be a poor brand tomorrow, and what is a poor brand today may be a good brand tomorrow. Just a change in management at an oil company might affect the quality of motor oils produced.