Yes, the Canon EOS R8 is compatible with the Lexar Professional 1667x 64GB SD.
Lexar 1667x is UHS-II (V60 rated) with 120 MB/s write speed. Canon R8 requires UHS-II bus and 30 MB/s minimum (UHS Speed Class 3) for 4K30. 120 MB/s on a UHS-II bus satisfies both requirements.
“Canon EOS R8 Compatible Cards: SD/SDHC/SDXC with UHS-II and UHS-I supported; for movies use a card with ample performance (fast enough writing and reading speeds) per Canon online manual.”
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Confidence: inferred