Do the oily secretions from the skin that are not easily removed and appear when the fingernail scratches act as a barrier preventing water from reaching the skin during wudu or ghusl from janabah, and what is the ruling on repeating wudu if these secretions reappear?

The fatty secretions that lead to the accumulation of dirt on the body do not invalidate ablution, because this dirt originates from the body itself. Excluded from this is dirt originating from outside the body. Thus, dirt formed from body sweat does not harm the validity of ablution or ghusl, unlike what originates from dust.