[Brock]

using any product for its proper application cannot void the warranty. Toyota would have to prove that the use of Synthetic oil ALONE caused or led to the failure.
as long as you are using a respected oil and not something put together by your 2nd cousin, 3 times removed on your great aunts side that is a hobby chemist....then you will be good. LOL

if manufacturers are not using synthetic it is to save money...nothing else could be influencing them. they then would have to prove that every other manufacturer using the same oils from the factory are doing something different then they are.

last thing is they cannot dictate a specific product to be used unless they provide that product to you. My wifes car requires specific type oil....so the dealership does her fluid changes for free. Once they stop...they can no longer hold us to using that same oil as long as what we use is rated the same or higher.