There are, I understand, two sets of active optative endings: -οιμι, -οις, -οι, etc., and -ίην, -ίης, -ίη, etc.
Both JACT Reading Greek and Athenaze, when they introduce the optative, teach that uncontracted verbs have the οι endings throughout, while contracted verbs have the ίη endings in the singular and the οι endings in the plural. Looking around in sources I find online gives a more complicated picture. It seems that both sets of endings can occur interchangably.
I assume that the textbooks have chosen to introduce the most common arrangement. But is that so? How much more common is it than other variations? Where can I most often expect to see exceptions? Which ones? Does it differ between dialects? Poetry, prose? Attic, Koine?
Or have I misunderstood?