Objection 1. It would seem that the form of this sacrament should be expressed by way of assertion rather than of petition. Because all the sacraments of the New Law have a sure effect. But sureness of effect is not expressed in the sacramental forms except by way of assertion, as when we say: "This is My body" or "I baptize thee." Therefore the form of this sacrament should be expressed as an assertion.
Objection 2. Further, the intention of the minister should be expressed in the sacramental forms because it is essential to the sacrament. But the intention of conferring a sacrament is not expressed except by an assertion. Therefore, etc.
Objection 3. Further, in some churches the following words are said in the conferring of this sacrament: "I anoint these eyes with consecrated oil in the name of the Father," etc., which is in keeping with the forms of the other sacraments. Therefore it seems that such is the form of this sacrament.
On the contrary, The form of a sacrament must needs be one that is observed everywhere. Now the words employed according to the custom of all the churches are not those quoted above, but take the form of a petition viz.: "Through this holy unction, and His most tender mercy, may the Lord pardon thee whatever sins thou hast committed, by sight," etc. Therefore the form of this sacrament is expressed as a petition.
Further, this seems to follow from the words of James, who ascribes the effect of this sacrament to prayer: "The prayer of faith," says he (5:15), "shall save the sick man." Since then a sacrament takes its efficacy from its form, it seems that the form of this sacrament is expressed as a petition.
I answer that, The form of this sacrament is expressed by way of a petition, as appears from the words of James, and from the custom of the Roman Church, who uses no other than words of supplication in conferring this sacrament. Several reasons are assigned for this: first, because the recipient of this sacrament is deprived of his strength, so that he needs to be helped by prayers; secondly, because it is given to the dying, who are on the point of quitting the courts of the Church, and rest in the hands of God alone, for which reason they are committed to Him by prayer; thirdly, because the effect of this sacrament is not such that it always results from the minister's prayer, even when all essentials have been duly observed, as is the case with the character in Baptism and Confirmation, transubstantiation in the Eucharist, remission of sin in Penance (given contrition) which remission is essential to the sacrament of Penance but not to this sacrament. Consequently the form of this sacrament cannot be expressed in the indicative mood, as in the sacraments just mentioned.
Reply to Objection 1. This sacrament, like the others mentioned, considered in itself, is sure of its effect. yet this effect can be hindered through the insincerity of the recipient (though by his intention he submit to the sacrament), so that he receives no effect at all. Hence there is no parity between this sacrament, and the others wherein some effect always ensues.
Reply to Objection 2. The intention is sufficiently expressed by the act which is mentioned in the form, viz.: "By this holy unction."
Reply to Objection 3. These words in the indicative mood, which some are wont to say before the prayer, are not the sacramental form, but are a preparation for the form, in so far as they determine the intention of the minister.
Source: Summa Theologica (New Advent)