First of all, oil related failures are very rare. Secondly, the manufacturer (or his agent - in this case the stealership's service department) would have to prove that long oil change intervals caused the failure. Thirdly, denial of a warranty claim could only be made against a failure involving a part subject to motor oil lubrication - in other words the manufacturer or his agent could not deny a claim for a bad steering box or headlight switch based on your extended use of engine oil. Just their saying extended use of motor oil contributed to a lubricated engine part failure is not enough - that's the point at which they'd have to meet the legal requirement of proving their claim. From a practical standpoint, they would say so and there the matter would drop unless you'd be willing to pursue litigation. If you do decide to embark on extended oil change intervals while under the powertrain warranty, better figure on on UOAs at EACH oil change for a running record of how the oil held up. Acceptable levels of wear metals and a reasonable TBN would be hard for Toyota's experts to argue with in court. The question comes down to: how far, and to what expense, are you willing to push back, if Toyota or the dealership butts heads with you if the very unlikey were to happen? There's only one person in the world who can answer that question for you.