Yes. It's informal. It might be more accurate to say that "I wish I were" is formal than to say that "I wish I was" is informal! "I wish I was" is very common, probably much more common than "I wish I were." Here are my suggestions. Keep using "I wish I were" yourself. Don't be surprised when you hear "I wish I was." But don't correct anybody who says "I wish I was." "Were" is a rare example of the subjunctive in English. In this example, "were" is supposed to be "good English." It's used because you are expressing a condition contrary to fact. "I am not rich, but I wish I were rich." This is a case where the usage of "was" is so common that it seems silly to call it wrong. I do use "I wish I were" myself, and "careful" people use it, particularly in formal writing. The language is what people speak. This is particularly true in English. There is no official authority for English and English dictionaries are traditionally compiled "on historical principles," recording what the language _is_, not what it should be. As far back as 1921, in "The American Language," H. L. Mencken complained that "All signs of the subjunctive, indeed, seem to be disappearing from vulgar American." In other words, he would have called "I wish I was" was wrong--"vulgar!" Yet he would have acknowledged that the use of "were" was "disappearing."

Alina, You are right. Grammar books say that 'were' is the correct option but we say 'was' in everyday speech. Victoria

Were is correct! ‘Was’ is used because half the time it’s a grammatical mistake (I feel like subjunctive verb changes are the biggest mistakes we American English natives make) and the other half of the time it’s just very informal and said out of laziness of changing the verb (I know it sounds weird but I’ve definitely said ‘was’ when I knew I should have said ‘were’ simply because I thought of ‘was’ first and was too lazy to change it in my head 😅)