In British English, you can either say 'I wish I was’ or 'I wish I were', which is rather formal. In American English, you should say 'I wish I were'. Also, "I wish i was" is most commonly heard/found in teenagers' conversations as well as in songs, but in today's natural usage based on "Descriptive Grammar (DG)", both are naturally used and are as well acceptable. However, here is another point that must be taken into consideration: a person, whose age is above 19 years, would use "were" because I already mentioned that "was" is mostly heard/found in teenagers' conversations and in songs. Therefore, for the purpose of this matter, let me exemplify it further as in the given examples:
