Is it permissible for men to use hair serum made of oils and vitamins after showering, and does it affect ablution (wudu') when wiping over the head?

If the oil has a solid mass, then it is considered an obstruction that prevents the validity of ablution. However, if it does not have a solid mass, it is not an obstruction.

Sheikh Ibn Uthaymeen says: "If the oil is solid and prevents water from reaching [the skin], then it must be removed before purification. But if it does not have a solid mass, then there is no harm in performing ablution, and one should pass their hand over the limb when washing it, so that the water does not slip off it."