It says in Masechta Tomid 2:3 that they used approximately 8 seah on Shabbos. Why does it say Omed with an Ayin which means standing rather than with an Aleph which means approximately?
You ought to have given the reference as 2:5, not 2:3. I wonder why you quote the 8 Se'ah on Shabbos and not the 4 Se'ah on weekdays that precedes it.
As for your question, the Bartenura comments that "Omed" with an "Ayin" is synonymous with "Omed" with an "Alef". No doubt, his comment is based on the principle that the two letters are interchangeable.
be'Virchas Kol Tuv,
See also Berachos 32b, "d'Vei R. Eliezer ben Yakov [sometimes] interchanged Alef and Ayin." Although in Yoma 16a we are taught that the Stam Mishnayos of Midos are R. Eliezer ben Yakov, but the Stam Mishnayos of Tamid contradict them, nevertheless, the Gemara's conclusion there (17a) is that the Mishnayos of Tamid do not contradict Midos, and may have been authored by the same pen.
By the way, in Zevachim 58a the Mishnah is quoted as b'Omed with an Alef.