Hello Bhante Dhammanando and Bhante Pesala,
Many thanks for your comments. I understand that all the eight were taught by the Buddha under either ten wholesome conducts or the noble eight-factored path. Sorry I didn't make my point clear. My point is that the Buddha didn't teach them as the eight precepts
On one hand, they might confuse with the eight monastic precepts set up by the Buddha, and they should probably not be offered to the lay practitioners as an alternative
to the eight monastic precepts.
On the other hand, those who can really observe all these 8 "precepts" would have already perfected their sila (right speech, right action, right livelihood). This is not very practical (and could dilute the lay practitioners effort on the five or eight precepts set up by the Buddha), since even stream winners could still have "evil conducts" and have not yet perfected their sila (they have just entered the N8P to perfect their speech/action/livelihood), to my understanding:
Sn 2.1 PTS: Sn 222-238
Ratana Sutta: The Jewel Discourse
"With his gaining of insight he abandons three states of mind, namely self-illusion/self view (Sakkaya-ditthi), doubt (Vicikiccha), and attachment to sila (precepts/virtues) and rites/rituals (Silabbataparamasa), should there be any. He is also fully freed from the four states of woe, and therefore, incapable of committing the six major wrongdoings.
This precious jewel is the Sangha. By this (asseveration of the) truth may there be happiness.
"Any evil conduct he may still do by deed, word
or thought, he is incapable of concealing it; since it has been proclaimed that such concealing is impossible for one who has seen the Path (N8P). This precious jewel is the Sangha. By this truth may there be happiness. ..."