The (accepted) answer by RiMMER is hardcore conservative and is correct only from that point of view - though the added quotation from FumbleFingers goes a long way towards fixing that.
English has been undergoing a fast process of simplification for centuries, and obviously it's not over yet. The English subjunctive was once different from the past tense for many verb forms, but due to phonological changes (and presumably a resulting process of regularisation for less frequently used verbs), the two have converged almost completely. At the moment, past tense was vs. subjunctive singular were is the only verb form for which it is even grammatical to make this distinction.
It is not surprising that we are already seeing the last phase of this convergence in action. You can see here that at least in the case of "I wish I was/were", English literature is about equally divided between the old-fashioned were and its new-fangled replacement was - and has been so for the last 300 years! Proper linguists, as opposed to people who just like telling others they are wrong, have drawn the conclusion that both variants are equally valid.
Even if you don't like subjunctive was, you should probably think twice before accusing Lawrence Sterne ("I wish I was with you, to do these offices myself, and to strew roses on your way"), Alexander Pope ("I wish I was as sure they would study to serve you") or Horace Walpole ("I wish I was less indifferent, for the sake of the few with whom I correspond, your lordship in particular, who are always so good and partial to me, and on whom I should indubitably wait, were I fit to take a long journey") of writing ungrammatically, and be it only in letters.
The Walpole example is interesting in that it uses first was, then were, and in a way that does not appear accidental to me. Simplifications of grammar sometimes first become more generally acceptable in frequently used idioms. And where Walpole used were, the sentence might have become confusing had he substituted was.
I have excluded direct speech and letters in novels from this sample on the grounds that they might be ungrammatical by design. It makes perfect sense that in the 17th century, subjunctive was should for some time have been associated with the uneducated and therefore should have featured in the speech of workers and letters written by women like Polly Darnford in Samuel Richardson's "Pamela". ("I wish I was with you for a Month, and all their Nonsense over without me.") More than two centuries later and in a more democratic society, it doesn't make so much sense any more.
In the few cases where it can prevent ambiguity, there is much to be said for were even today. As a German native speaker I personally tend to use it anyway because it closely mirrors the distinction made in German, where the pair war/wäre corresponds to was/were. But those who grew up in an environment where subjunctive singular were has disappeared are perfectly justified to use was instead.
PS and partial correction: Janus Bahs Jacquet's comment prompted me to actually look some things up instead of relying on my memory. Turns out that subjunctive isn't really a good name for the phenomenon because it's already used for something entirely different. (E.g. the word be in be that as it may and lest there be any doubt.) The Cambridge Grammar of the English Language calls this mode where were can be used instead of was the irrealis. There is a characteristically unkind Language Log post on the distinction and a related matter by linguist Geoffrey K. Pullum, one of the authors of that grammar.
Anyway, the point of view of the Cambridge Grammar of the English Language on the was vs. singular were phenomenon is that the choice is a matter of formality. This also explains why irrealis was doesn't go with inversions. Inversion is so formal that we just don't expect an informal expression in its immediate vicinity.
