What is the ruling on wiping over hair that has been treated with oil or henna for nourishment during wudu for prayer in the Maliki school of thought?

If grease and henna have a substance that prevents water from reaching the skin, then ablution is not valid with their presence, and they must be removed. However, if they do not prevent water from reaching the skin, they do not affect the validity of ablution. It is permissible to wipe over them out of necessity, such as for medicinal purposes, if there is fear of harm by removing them. But if the henna is not for necessity, then wiping over it is not sufficient.