I'm no Synlube fan, but respectfully, your statement about the wty is not correct. All US new car wtys are governed by the federal Moss-Magnuson Warranty Act (MMWA). Under the MMWA, a mfr has the burden of proving that something you (or a 3rd party) did was the cause of a claimed failure, instead of a problem with the car. Whole wyts can't be voided, or claims denied as a "penalty" for not using the spec-ed oil. If an owner uses Synlube and has an engine failure, the mfr would have prove that the Synlube caused the problem. Even if they were able to meet their burden (which would probably require engineering testimony), the wty would still be in full force for the rest of its term as to other problems.
Example: you use 80 wt oil and it causes your engine to blow up. You take the car in for the blown up engine and a door handle that has simply fallen off, not because of any abuse. The maker easily proves that you caused the engine failure, and will win on that claim, but they still have to fix the door, as your wty is still in full force, despite your trashing the engine.