But I wonder what changed internally. Some people run 5/30 without issues but that's also something I wouldn't do.
I am saying Toyota is hedging their bet and the recommend OCI is purely based on synthetic vs conventional and nothing to do with the engine internally.
If 5W-30 was only available in synthetic, they would recommend 10k change. However, since 5W-30 is also available in conventional (0W-20 isn't), Toyota is playing it safe and avoiding any potential litigation loop hole.
That's how Toyota's statement reads to me.