The phrase dharma sanātana does occur in classical Sanskrit literature, meaning "cosmic order".
மந்திரம் லௌகிகம் | தந்திரம் விஜ்ஞானம் || தர்மம் ப்ரஹ்மஸ்வருபம் | ஸுன்யப் பிரச்னம் போதிஸத்த்வம்
[vedic hymn/rituals (மந்திரம்) are for worldly things; scientific material or techniques (தந்திரம்) in vedas are for understanding universe. Truth/laws (தர்மம்) are the forms or views of ultimate reality. Only by questioning with open or empty mind, one can attain enlighytenment]. Openness or egoless state is the way to the attainment of perfect knowledge or enlightenment
Attributed to Salivahana as contents of all scriptures.
Buddhists used it as their main doctrine.
புத்தம் சரணம் கச்சாமி | தர்மம் சரணம் கச்சாமி | சங்கம் சரணம் கச்சாமி
Respects to Buddha (saints and enlightened ones), Dharma (cosmic order or duty.) and Sangha (spiritual community promoting order)
Sanatana dharma, is being used to denote the “eternal” or absolute set of duties or religiously ordained practices incumbent upon all Hindus, regardless of class, caste, or sect. Different texts give different lists of the duties, but in general sanatana dharma consists of virtues such as honesty, refraining from injuring living beings, purity, goodwill, mercy, patience, forbearance, self-restraint, generosity, and asceticism.
Svadharma, one’s “own duty” or the particular duties enjoined upon an individual according to his or her class or caste and stage of life.
Each sect define their own karma and there is no point in arguing which Dharma is the best. Every dharma has its own value, just as every one's duty. Each caters to certain needs and fulfills certain aims.