I know that we don’t usually bring back dead threads, but regarding the question of whether Rashi’s identification of oats as one of the five grains is accurate, just published today is a description of what was found at an ancient archeological site in Israel:
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0131422
Avena sterils is a wild relative, possibly the ancestor, of the common domestic oat Avena sativa.
Rashi may have had a tradition that has been since lost, or he may have been writing with Ruach HaKodesh. But in any case, the claim that oats were not eaten in the Land of Israel in ancient times has now been disproven. It is also interesting that two of the other grains found were forms of wheat and barley.