General motors provides my powertrain warranty for the first 5 years or 60k miles of the truck's service life. That's why I don't necessarily believe them on anything, but I do follow their spec in order to be blameless and pure in the event of a major warranty claim.
Amsoil 0w20 is going for $7.99/quart on their website.
AcDelco 0w20 Dexos Gen 2 Full Synthetic is $4.25/quart on Rockauto.
Mobil 1 0w20 AFE Dexos Gen 2 Full Synthetic is $6.32/quart *before rebate* on Rockauto.
Common sense, as they say, isn't so common. If I keep my truck for 200k miles, which would be a record for me, and I can lower the cost of oil changes by just less than 50%, while meeting the exact oil spec, why would I buy a product which is not only more expensive, it doesn't carry the certification. Meanwhile, the manufacturer says they're "saving me money", which, based on comparable product which meets the manufacturer's requirements, just isn't true.
But by all means, if someone wants to pay more for motor oil because they like the way the bottle looks, and doing so makes them feel both virtuous and righteous, I suppose it's cheaper than therapy in the long run...