Trying to be peaceful here and not cause any discourse. It's your vehicle, put whatever you want in your crankcase.
1. If the vehicle is under warranty, use the oil spec'd by the manufacturer and change by OLM or manual recommendations.
2. Out of Warranty: Use whatever you want and sleep well at night.
3. If you want extended OCI's, do some research and pick the best oil for your needs. Wether it is a Grp III, IV, or V oil, it is up to you.
Since I am conservative, I use a Grp III synthetic and change every 5-6K miles. I follow the severe service schedule. The OM says Mineral (conventional) preferred, change every 3750. I use API spec'd oil and have no worries.
It should be up to the owner to put in whatever they want. They will have to deal with the dealer, manufacturer, or oil distributor/refiner, if there is an oil related problem.
That is my .02 bash away if you want.