It's one of those questions that can easily start a fight in a pub full of grammarians, but I would say it isn't set in stone as so many native speakers don't use it. Also, I have heard people who say 'if I were' using 'I wish I was'; it seems less stony.
Is it set in stone that the verb 'wish' should always be used with 'were' and not 'was'?
I wish my class were not (weren't) so boring [seen in an AE textbook].
In England it would be more common to say:
I wish my class was not (wasn't) so boring.
I think I have subjunctivitis.
I wish I were as clever as you!
British English is not as concerned aboput the subjunctive as American English. And not all Americans use it either. That said, you will sound uneducated to many Americans if you use the indicative with wish. The indicative probably sounds more normal in Britain.
I think it would possibly sound more normal in British English as I do hear people who use the subjunctive in conditionals using the indicative with wish, though, as always, formal usage favours the subjunctive.