It is simialr to a conditional, so some prefer 'were'. However, the use of 'was' is accepted by most, and many who say 'If I were' say 'I wish I was'. It depends on viewpoint about the acceptability of 'was' in conditional clauses. Exam boards like the University of Cambridge accept it, while GMAT doesn't. To be on the safe side, it might be better to use 'were'.