Oil cheaper than either of them will get the job done, nevertheless Quaker State is the bargain brand of Shell and Pennzoil is the higher priced brand. And brands is what they are neither of them is a company, Shell Oil is the company. The brand positioning is marketing.
For the ‘motor oil warranty’ though I try not to laugh about them. All oil company warranties are useless. You are going to stick to one brand for 10 years. Now the warranty program ends at 10 years or 300,000 miles whichever comes first. So let’s play along you are at 10 years and 1 day and your car has not reached 300,000 your engine repair warranty ends. Most 10 year old cars have more like 150K miles not 300K. You would have to drive 30,000 miles a year for 10 years to get the 300,000 miles of engine coverage. But the nearly a sucker bet part of it is they say well if you are a normal driver you have 20 years to reach 300,000 miles for the ‘cash bonus’. But to stay in the program you had to use Quaker State synthetic for 20 years and do every maintenance step in your owner’s manual for 20 years not just oil changes. Belts, hoses, cololant changes all by the book for 20 years or 300,000 miiles. And you better have 20 years of receipts to show them. That is if they don’t just end the program in the next 20 years.