amazon's algorithms can price things strangely, because it appears (speculation) to take real-time demand and supply into account. I note the amazon page says only 9 left in stock. Pennzoil doesn't seem to prioritize marketing a 5qt 20w50 conventional jug (or 1 qt bottles for that matter, though 1 qt is a bit more common), you rarely see it at common physical retail stores. I can't find it as even available to order (not including third party sellers) on walmart, target, oreilly's, autozone, advance auto parts, or napa. But since there's not a lot of alternative places to buy a 5 qt pennzoil 20w50 bottle, plus the stock issue, that could be why it's priced the way it is
In contrast, a 5qt 20w50 Castrol GTX conventional jug is pretty to easy to pick up at any brick and mortar auto parts store and wal mart (the wal mart price is the same as amazon). The only 5qt 20w50 conventionals I see at stores is Castrol GTX, Valvoline white bottle, and whatever the store brand is
I'm not going to comment on the relative quality of castrol vs pennzoil, but I will say the Pennzoil is well over-priced just in terms of Pennzoil. Napa has 1 qt Pennzoil 20w50 bottles for $4.50 which appear to be the same formula (marked down from list price of $7.60 -- I understand 5 qts at the original $7.60 price would be in line with the amazon price, except you always see 5 qt oil jugs priced at a discount to to the 1 qt bottle price, so it's not really in line)
https://www.napaonline.com/en/p/PZR550035074?intent=true
Pepboys has 1 qt pennzoil 20w50 at $3.80
https://www.pepboys.com/pennzoil-conventional-motor-oil-20w-50-1-qt-/product/1103315
edit:
Heck, I see under the comparative products on the amazon page that they have a 6 pack of 1 qt pennzoil 20w50 bottles (6 qts) for only 2 dollars more than the 5 qt jug ($36.68)
https://www.amazon.com/Pennzoil-550...dp/B00CBT5J36/ref=psdc_15719391_t1_B01M4KQQLI
