It depends on how the car company supports the dealerships. While the dealer gets paid less for warranty work than for "retail" customer work, they still get paid. This is not as bad as having an angry former customer. While we think that the car company needs to prove that the owner failed to do the required maintenance, just stonewalling the customer works in most cases. You need your car to get to work, etc. Rather than fight, hire a lawyer and an expert witness for a repair worth a few thousand dollars, just pay and go away mad.
As noted above, any warranty denial can only be on a part where the service to that part is in question. Failure to change the oil must not be claimed as cause for a heater fan failure, etc.
If I was taking a car in for a warranty claim on an oil lubricated part, I'd draw an oil sample for testing first. I'd have an independent licensed auto repair shop pull the sample, charge me for their time, and have the paperwork. I'd insist that the dealership pull another sample while I watched, and document, maybe photograph, this. The oil sample may show sky high wear metals--there's a failure--but the oil should be OK with acceptable levels of oxidation, viscosity, no fuel or coolant dilution unless that's the reason for the problem, etc.