For any reason that voids your warranty the dealership would have to have very exact evidence that ties either modification or neglect directly to damage done to the vehicle. Even if you put on some aftermarket intake that relocates a sensor, and that sensor goes bad they would need to prove that the installation of the intake caused that failure. They would still not be able to void your engine warranty, they just may not warranty that single sensor. In the US the consumer is 100% allowed to self maintain the vehicle as well. There is no specific language either saying you need to provide proof of oil changes with receipts or photo evidence etc... In the event of a blown motor, the dealership would need to prove by way of non anecdotal evidence, that the damage was caused by neglect or misuse.