Is oral sex from a woman to a man considered Zina (fornication/adultery) or a lesser sin, or is Zina limited to penetration only?

Do not look at the smallness of the sin, but look at the greatness of the One Whom you have disobeyed, and know that there is no minor sin with persistence, and no major sin with seeking forgiveness. Minor sins are not all of the same degree; the sin of touching is greater than the sin of looking, and lying intimately is greater than touching, and penetration is a major sin and greater than mere intimate lying.