Under normal conditions the middle of the face produces more oil than is necessary, and is enough to supply the outer part of the face with oil when it slowly disperses. The T-zone produces more oil than other areas as a protective measure, as the middle of the face will come into more contact with external factors (food, weather, etc. ) and there are more sensitive body parts around which to protect (eyes, nose, mouth). The oil should even out steadily and not be noticeable.
Sebum, in normal levels, should be unnoticeable on the skin. However, your skin can overproduce sebum, with positive and negative consequences. One positive side is that oily skin tends to be more resistant to developing fine lines and wrinkles. The extra oils on the skin are even better at keeping water from evaporating away. Hydrated skin and a strong oil defence barrier keep skin processes functioning as they should be, and the skin stays plump and thick.