A priest I requested kindly to bless salt and oil did not use the prescribed prayer but used his own, is the blessing valid?

About the rite: It is NOT found in the "Book of Blessings." This fact should be a red flag for anyone (other than a priest) wanting to perform the rite for blessed Exorcised Salt, Water, and Oil. It is from the Roman Ritual. These prayers can be used without a special permission by any priest. But they are not to be said by deacons, religious, or the laity!
The Priest vests in surplice and purple stole and must follow the rite, word for word. In other words he cannot use his own prayers for any reason. I'm not sure if an alb can be worn instead of a surplice. I'll have to ask my friend (an exorcist priest) about this.

The two first prayers are used for blessing salt. To bless water, if salt has already been blessed (prior to the water), the two prayers do not need to be repeated. Blessing of oil: in the end an aspersion (sprinkling) with holy water is required.

I'll post more later on about using these sacramentals as I get time.

Blessing and Peace

Deacon Del

Regarding the rite for BLESSED EXORCISED SALT, WATER and OIL.

If you are expecting the blessing for EXORCISED SALT, OIL, and WATER, the blessing MUST be the prescribed EXORCISM BLESSING rite. Otherwise you have only blessed salt, oil and water.

Only a priest can use and say this blessing. As a deacon, I asked this question to our auxiliary bishop - if I could perform this rite. He was not sure and said that he would get back to me. Then I asked one of our priests, who is an exorcist, and he said: absolutely NO! The rite for exorcism of salt, water and oil is not a standard blessing and is ONLY reserved for priests and bishops. Later when I spoke with the bishop he agreed that only a priest or a bishop could perform this rite.

If a priest blesses salt and oil, but does not use the prescribed prayer, but used his own wording, is the blessing valid?

The short answer is yes.

But there is a caveat to my response.

Rome allows objects to be blessed by deacons, priests and bishops. The traditional method would be to use the prayers contained in the Rituale Romanum, making a sign of the cross over the object to be blessed and sprinkling it with holy water.

A priest saying a few word in blessing, while employing the sign of the cross is sufficient to make the blessing valid.

Sometimes the clergy member blessing something will say a few words of their own before giving the words of blessing, and sometimes the cleric will simply say a silent prayer over the object and make the Sign of the Cross over it. Any of these methods is a valid blessing done with the intent of the Church. - Q & A: Blessing Objects and Things

Not using the traditional formula will weaken the efficaciousness of some prescribed blessings, especially if a minor exorcism is involved.

Some priest chose to do such simple blessings since there is no holy water on their person.

Some of the formulae contained in the Rituale Romanum are in fact minor exorcisms. In these cases the ritual must be followed word for word, with the sprinkling of holy water at the appropriate moment.

For this reason alone, I carry a Pocket Holy Water Sprinkler with me when attending church. I also bring with me a hard copy of the prayers I desire the priest to employ.

Some priests will no doubt be uncomfortable with this, but more traditional priests will not have any problem.

There is a Benedictine Abbey near my residence and the abbot carries an Aspergillum as seen below to bless objects (usually medals), but employs his own words of blessing. If a particular object have a blessing listed in the Rituale Romanum, I make a hard copy to take with me when getting it blessed. Most priests do not carry the Book of Blessings with them, but some do.

Pocket Holy Water Sprinkler Made of Stainless Steel with Holding Case

As Peter showed in his comment, some blessings are in fact minor exorcisms and just not simple blessings. In theses cases the prayers prescribed by Rome must be followed.

Minor Exorcisms

According to Catholic belief demons or fallen angels retain their natural power, as intelligent beings, of acting on the material universe, and using material objects and directing material forces for their own wicked ends; and this power, which is in itself limited, and is subject, of course, to the control of Divine providence, is believed to have been allowed a wider scope for its activity in the consequence of the sin of mankind. Hence places and things as well as persons are naturally liable to diabolical infestation, within limits permitted by God, and exorcism in regard to them is nothing more that a prayer to God, in the name of His Church, to restrain this diabolical power supernaturally, and a profession of faith in His willingness to do so on behalf of His servants on earth.

The chief things formally exorcised in blessing are water, salt, oil, and these in turn are used in personal exorcisms, and in blessing or consecrating places (e.g. churches) and objects (e.g. altars, sacred vessels, church bells) connected with public worship, or intended for private devotion. Holy water, the sacramental with which the ordinary faithful are most familiar, is a mixture of exorcised water and exorcised salt; and in the prayer of blessing, God is besought to endow these material elements with a supernatural power of protecting those who use them with faith against all the attacks of the devil. This kind of indirect exorcism by means of exorcised objects is an extension of the original idea; but it introduces no new principle, and it has been used in the Church from the earliest ages. - Exorcism (Catholic Encyclopaedia)

I get olive oil blessed each year if possible during Eastertide according to traditional old rite, in Latin, by priests who say the Mass of the Ordinary Form and generally have no problems. Know your priests first. Some will be uncomfortable with it and many do not even read Latin, in which case one can always find a vernacular translation somewhere online.