Obagi Nuderm kit with Obagi or generic tretinoin?

Do you have to use Obagi branded tretinoin? I was told yes as it complements Obagi blender, however the price of obagi tretinoin is significantly more expensive than generic tretinoin creams. Can you use tretinoin under the eye for dark circles? How long should a normal size Nuderm Kit last? I was told by 6 weeks I should need a replacement of the Obagi Clear and Obagi tretinoin but I still have at least half left of both items!