Memorized from a written set of stories/instructions.Even in our most modern communities. My son is in the Marines and they memorized their 11 General Orders, plus the Rifleman's Creed in their boot camp. They had to recite it out loud everyday until they had it down perfectly. So yes, oral tradition works. You may choose not to believe it, but look it up for yourself if you don't want to believe me.
That's not oral-- that's writing. Someone, somewhere has written it down, or it **does** change with the generations.