I have a sample toyota owners manual, here it says "Recommended" so it might not be as strict as I thought. Also there is no notice not to use other grades. For example, for spark plugs it strictly notices not to use other than iridium etc.
In general, warranty is against manufacturing defects, not act of god, or misuse, or neglect etc. Here is a good article on it:
Modifying your new car won’t automatically void your manufacturer’s warranty. But it may cost you depending on the mod. Here is what you need to know. I got a few questions the other day about vehicle modifications and whether a manufacturer can void your warranty over them after I mentioned the...
lehtoslaw.com
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