Whether the oil ought to be consecrated? St. Thomas of Aquinas

I answer that, Some hold that mere oil is the matter of this sacrament, and that the sacrament itself is perfected in the consecration of the oil by the bishop. But this is clearly false since we proved when treating of the Eucharist that that sacrament alone consists in the consecration of the matter (2, 1, ad 2).

We must therefore say that this sacrament consists in the anointing itself, just as Baptism consists in the washing, and that the matter of this sacrament is consecrated oil. Three reasons may be assigned why consecrated matter is needed in this sacrament and in certain others. The first is that all sacramental efficacy is derived from Christ: wherefore those sacraments which He Himself used, derived their efficacy from His use of them, even as, by the contact of His flesh, He bestowed the force of regeneration on the waters. But He did not use this sacrament, nor any bodily anointing, wherefore in all anointings a consecrated matter is required. The second reason is that this sacrament confers a plenitude of grace, so as to take away not only sin but also the remnants of sin, and bodily sickness. The third reason is that its effect on the body, viz. bodily health, is not caused by a natural property of the matter. wherefore it has to derive this efficacy from being consecrated.