What is the criterion for wiping over the head, and is it permissible to wipe over the outer part of the hair without the water reaching the scalp? And what is meant by a "jirm" (substance/film) of oil or cream that invalidates ablution, and what is its criterion?

Wiping the head during ablution is an obligatory act, based on the Almighty's saying: "And wipe over your heads." However one wipes it, it suffices. Among the recommended methods of wiping is for the Muslim to place his index fingers at the front of his head and his thumbs on his temples, then pass them to the back of his neck, and then return them to the front. Wiping over the visible hair is sufficient, and it is not necessary to reach the roots. Any substance or barrier that invalidates ablution is anything that prevents water from reaching the skin. However, traces of henna or liquid oil that do not prevent water from reaching the skin do not invalidate ablution.