True, Rav Schachter doesn’t believe that oats is one of the five grains but the MAJORITY of gedolai poskim do hold it is… I think its told of Rav Moshe that he said that even if they were to bring a million raayas that oats is not one of the 5 grains, it still is because that has been our mesorah.
I’ve heard stories with Rav Shlomo Zalman that he took upon himself the chumra of being choshesh its not mezonos but then Rav Elyashiv told him not to and then he stopped. When asked he told others to just do like the our mesorah and don’t be choshesh.