In Latin, it's "opera," but in English, "opuses" is OK.
I'm curious as to why a farewell message is called an "opus" on this board--perhaps it indicates that all of a member's postings are now a completed work. However, I would suggest that "valediction" be used instead of "opus."
Good luck. It'll be adopted PDQ. Just like my idea of replacing "racism" with "ethnocentrism", which is the correct description.
In any event, what is plural of "Lexus", while I'm in the process of building up my stable?