Or do u need initiation?
Mantras from Smritis don’t need initiation. Not sure where this mantra falls?
Om ithyagre vyaahareth nama ithi paschath. Narayanasa ethyuparishath. Om ithyekaksharam. Nama ithi dhwe akshare. Narayanayethi Panchaksharani. Ethadwai Narayanasyashtaksharam padam. Yoha vai Narayanasya ashtaksharam pada madhyethi. AAnapabroova sarva mayurethi. Vindathe Prajapathyam rayasposham gowpathyam thatho amruthathwamasruthe thatho amrutha masnutha ithi. Ethath Sama Veda siro adithe. 3
Tell “Om “ first and then tell “Nama” After this tell “Narayana”. There is one letter in “Om”. There are two letters in “Nama”. There are five letters in “Narayana.” Together is formed the eight lettered “Om Namo Narayana”. He, who tells these eight letters, attains full life without any blemish. He would attain salvation after becoming the lord of the people and be blessed with lots of wealth, lots of cows and all other forms of wealth. Thus is read the Upanishads of Sama Veda.
The above are quoted from the Narayana Upanishad which is linked with the Krishna Yajurveda.
So, it is highly unlikely that the mantra (which is part of a Vedic literature) won't require initiation. Om itself is the first mantra which obviously requires initiation.
Your own question (answered by me) is related here (about whether Om japa is mantra japa or Nama japa): Is chanting “OM” naam japa or mantra japa?
Vedanta Desika discusses in detail about this aShTAkShara mantra in the mUlamantrAdhikara chapter of the Srimad Rahasyatrayasara.
In the very first introductory verse, he summarizes the mantra as -
tāram pūrvaṃ tadanuhṛdayaṃ tacca nārāyaṇāyeti
āmnāyoktam padamavayatāṃ sārtham-ācārya-dattam
The bolded portion means that this mantra is to be received from a teacher along with its meaning (implied and not without).
You can read the English translation of this work here.
There are many prohibitions and restrictions on the recitation of mantras by sudras, women and people without deeksha in various Smritis. But they all date back only to the Middle Ages.
There are ancient Sruti pramanas outweighing all these things. For example Shukla Yajurveda Madhyandina Samhita 26.2.
The realities of today's Hinduism are such that long before they get full deeksha, which is usually done in adulthood, Hindus begin to chant the mantras of their sampradaya. If you have heard the traditional performance of a mantra, on video or audio recording, then you can safely repeat this mantra. Because the principle of oral transmission is indirectly observed. You have heard a mantra from a brahmin or popular siksa guruji. Today in India, as well as in the Hindu countries of Southeast Asia, many practice mantra japa in this way.
It's only 1268-1369. And relevant to Ramanuja's sampradaya. But what about those who are guided by more ancient texts and love Narayana-Vishnu outside this particular sampradaya? I have personally heard this mahamantra also from smarta and vaikhanas followers. And I know that they did not receive any special initiation into it.
The prohibition on pronouncing Pranava by non-double-born and women is NOT in the ancient Sruti themselves (in the Vedas, Brahmanas, Aranyakas, Upanishads). He appears only in medieval Puranas, Agamas, Tantras, Dharmashastras, etc. later texts of the category Smriti. Moreover, for example, Aytareya Aranyaka 23.6 states that Pranava is necessary to give power to mantras. Also among the Vedic Rishis were women.
It is not possible to gain spiritual knowledge without a guru. Initiation is a must.
BRAHMO: "Is spiritual knowledge impossible without a guru?"
MASTER: "Satchidananda alone is the Guru. If a man in the form of a guru awakens spiritual consciousness in you, then know for certain that it is God the Absolute who has assumed that human form for your sake. The guru is like a companion who leads you by the hand. After the realization of God, one loses the distinction between the guru and the disciple. 'That creates a very difficult situation; there the guru and the disciple do not see each other.'1 It was for this reason that Janaka said to Sukadeva, 'Give me first my teacher's fee if you want me to initiate you into the Knowledge of Brahman.' For the distinction between the teacher and the disciple ceases to exist after the disciple attains to Brahman. The relationship between them remains as long as the disciple does not see God."
The Gospel of Sri Ramakrishna, Chapter 10, The Master with Brahmo Devotees (II), April 22, 1883
Why is initiation necessary?
"It is extremely difficult to teach others. A man can teach only if God reveals Himself to him and gives the command. Narada, Sukadeva, and sages like them had such a command from God, and Sankara had it too. Unless you have a command from God, who will listen to your words?
"Don't you know how easily the people of Calcutta get excited? The milk in the kettle puffs up and boils as long as the fire burns underneath. Take away the fuel and all becomes quiet. The people of Calcutta love sensations. You may see them digging a well at a certain place. They say they want water. But if they strike a stone they give up that place; they begin at another place. And there, perchance, they find sand; they give up the second place too. Next they begin at a third. And so it goes. But it won't do if a man only imagines that he has God's command.
"God does reveal Himself to man and speak. Only then may one receive His command. How forceful are the words of such a teacher! They can move mountains. But mere lectures? People will listen to them for a few days and then forget them. They will never act upon mere words.
"At Kamarpukur there is a small lake called the Haldarpukur. Certain people used to befoul its banks every day. Others who came there in the morning to bathe would abuse the offenders loudly. But next morning they would find the same thing. The nuisance didn't stop. (All laugh.) The villagers finally informed the authorities about it. A constable was sent, who put up a notice on the bank which read: 'Commit no nuisance.' This stopped the miscreants at once. (All laugh.)
"To teach others, one must have a badge of authority; otherwise teaching becomes a mockery. A man who is himself ignorant starts out to teach others — like the blind leading the blind! Instead of doing good, such , teaching does harm. After the realization of God one obtains an inner vision. Only then can one diagnose a person's spiritual malady and give instruction.
The Gospel of Sri Ramakrishna, Chapter 5. The Master and Keshab, October 27, 1882
What does Sri Ramakrishna mean in the above passage?
An analogy will be helpful to understand the meaning. Only a trained doctor can really diagnose one's physical malady and give a diagnosis. Similarly a Guru who has realized God will know one's spiritual malady and give remedial measures. Repeating a mantra without initiation is like self diagnosis and medication. It will not cure one's spiritual malady.