Well, your top layer is what gets the most exposure, and therefore the most damage. Do you rinse your hair with hot water in the shower? If so, you could be "cooking" all the moisture out of that section of hair with the heat. Also, do you often sleep or lean back on the frizzy parts? Any type of friction can rough up the cuticle, making it dry and frizzy. I would moisturize as much as possible, while avoiding cones and other build-up causing ingredients.
Also, you mentioned Burt;s Bees avocado treatment... did the dryness start after using it or before? I can't use that stuff because I have a heck of a time getting it out of my hair. If the problem started after using it, I would suspect that it might have built up. Try a baking soda scrub or a lemon-aid (conditioner and lemon juice), and scrub really well. Hope you get it figured out soon!
ETA: My brain is about to explode trying to figure out what SAO stands for. I'm sure I'm just being stupid, but please spare me.