The issue with the warranty is a moot point because the OP bought a LEXUS.
You're not going to have issues with warranty claims, as compared to other brands, especially if you're the original NEW purchaser (as compared to the guy who bought it used, with 1month left on the warranty).
They will give you the benefit of the doubt, or goodwill a repair to you and not make you "prove" you used a particular oil.
Lexus values customer satisfaction and loyalty to the extreme.
Taking the extreme flipside:
If you got a problem, and they really decide to deny you a warranty claim(when they suspect it's your fault), they will use some other made up reason that's not disputable and they will avoid getting in a he-said/she-said argument; so the issue of the oil won't make the difference.
(Meaning if you got denied, if you went back in time and had proof you used the 0w-20 it wouldn't have made a difference in the warranty outcome)
Don't worry about it, unless you truly have NOTHING else to worry about.