In the early days of the car which I consider to have ended in the early '70's with the advent of electronic ignition and emission controls the every six months tune up was considered a fact of life, couldn't get around it. This was the days when points had to be adjusted or replaced every 6-12K miles. (Those of you who don't know what points are, just bear with me.) Oil change intervals were 3k miles, transmission fluid 15k miles. That was with the technology of the times.
With every manufacturer it is the responsibility of the vehicle owner to properly maintain the vehicle at their own expense unless specified otherwise. In response to consumer complaints about what was considered excessive maintenance just for the sake of making money, the manufacturers started looking into ways to stretch the intervals. Changes in spark plug technology boosted those to 30k and in many instances 100k replacement intervals. Transmission fluid went to 30k, with some manufacturers spec'ing 100k under ideal conditions. (You still won't catch me going over 60k, though.)
Coolants went through a change as well, bringing us the infamous Dex Cool brown goop fiasco.
Engine oil operates under a whole different set of conditions than transmission fluid or coolant due to its close proximity to combustion. Engineers are engineers, not magicians. They can't change the conditions, they are what they are. But in response to those same demands, the dual service schedule was devised.
So what that means is some tightwad opens the book, sees the (alleged) ideal world service schedule, puts on their blinders and says 'Aha! I can go 7500 miles between oil changes, so sayeth the book.' But the book has no control over real world conditions. Contamination still happens.
So the consumer goes 7500 (this being the real world usually a bit more)between changes and at 60 or 70K encounters oil burning issues or even worse. Investigation reveals that the engine has sludge and goo. Of course it's the engine's fault. This is not made up; I have seen this first hand with every brand of vehicle I have worked with: Ford, GM, Toyota, Isuzu, Kia, Subaru, Mercedes etc etc etc.
Synthetics help but aren't the complete answer. It comes down to this: the most expensive oil change is cheaper than the cheapest engine. Which would you rather buy? FWIW, I change the Trooper's dino juice every 4500 or so. If it's race season meaning I tow more, every 3K.
One other thing: engine coolant temperature has a LOT to do with oil longevity. The 190 or so coolant temp is what the engineers have spec'd. A thermostat stuck open will keep the engine temps down and oil (and fuel) consumption will go up. I've had this happen to me!
By the way, as outlined by the Magnuson-Moss Act the dealership cannot 'take away your warranty'. Only the vehicle manufacturer can do that and they better have a damn good reason. If the dealership has reason to believe there is a non-warrantable reason for a failure, they MUST notify the manufacturers' rep who will then investigate and make the call.
Per the M-M, if a certain OEM maintenance part (such as a filter) is REQUIRED to preserve your warranty that part must be supplied by the manufacturer at no charge during the warranty period. Otherwise, a non-OE part can be used. BUT- if a failure can be shown to be as a result of the failure of that non-OE part then the manufacturer has the right to deny that claim.
If there is an aftermarket 'extended warranty' (actually a mechanical breakdown insurance policy, big difference!) involved then they can write the rules to suit themselves (up to a point).