Purity might be the same between brands, but what (if any) detergents are included and how well the oil holds up to thermal breakdown and turning into sludge are a different matter, altogether.
That being said ... store brand oils are kind of a "you get what you pay for" thing. It won't kill your car to put it in once in awhile ... but it's sort of like the difference between cheap beer and good liquor - both will do the job, but when things go sour (like, the next morning, or if you go over 3k miles before an oil change), I'd rather have a headache than the dry heaves and a case of "duck butt."
Pennzoil is not anything that most folks worth their salt would consider a "quality oil."
FWIW, I run Castrol GTX most of the time ... although I do throw in Valvoline from time to time if there's a sale that tickles my fancy.
Basic FRAM filters are poo, but I've always had good luck with their Tough Guard filters, and the grippy texture on the back makes removal later on MUCH easier. If Motorcraft would do the same with their FL1A filters, I'd probably switch.
Also, every oil is only going to be as good as the filter you're running. If you run some flimsy, cheap-arsed store brand filter (cut one open some time and give it a look, then compare it to a high-end filter, you'll see) then it wouldn't matter if you've got the best oil on the market in your engine - it will only hold so much dirt and sludge n' crap before it plugs up and either goes into bypass mode or starts releasing clumps of poop into your engine. If you prefer an analogy, think of the difference between a cheap-o filter and a quality one as the difference between a maxi-thin for "light days," and one of those super-duper absorbant maxi pads with wings and all that for "heavy days."
Bullshiit you cant kill an 300 I6, the ford 300 I6 can neva lose. 
