Is it necessary to remove the layer of accumulated grease on the armpit hair to ensure water reaches the skin during ghusl from janabah, and what is the ruling on prayer if this layer is not removed?

Scholars have differed regarding the obligation of washing flowing hair for someone performing ghusl, and the preponderant opinion is that it is obligatory. The oil present on the hair must be removed if it prevents water from reaching it; otherwise, the ghusl is invalid. If the oil cannot be removed except by removing the armpit hair, then its removal becomes obligatory. And whoever leaves an unwashed spot in their purification must wash it and repeat the prayer.