Adding to (while concurring with) MFS' comments -
(a) sometimes the OEM doesn't identify, successfully, all considerations at certification eg the Nomad forced landing following icing problems with an OEM FT crew.
(b) ongoing requirements for continuing airworthiness impose an obligation on the TC holder to modify instructions in the light of new knowledge gained from operations.
(c) AFM instructions and guidance do not derive from the OEM's folks sitting around a table drinking coffee and discussing the aircraft .. there is a wealth of test data behind the words of wisdom.
(d) and his critical point .. you ignore the TC holder's instructions and guidance at your potential peril.