"If I were... I would" is correct, and considered "better" English. If I were you, I'd use it.
However, "was" has been so common for so long that it is also accepted.
As far back as 1921, H. L. Mencken was complaining that 'All signs of the subjunctive, indeed, seem to be disappearing from vulgar American. One never hears “if I were you,” but always “if I was you...."