"If I were you" is grammatically correct, by strict standards. Grammatically, it is called the subjunctive mood. However, in any informal situation, especially spoken, you would most likely hear "If I was you". That tense is the simple past. Only an English teacher would correct you.

Yeah, I agree with both of the above. In Australia, people can be pretty lazy and you hear both, neither is not much more common than the other.

Here in the United States, the so-called "subjunctive" is still very strong. I have read that even little American children often say "If I WERE you, I WOULDN'T do that." The theory is that they hear it so often that they just naturally repeat it in their speech. So if you are interested in American English, you might consider learning more about the subjunctive. You use it when you are NOT talking about reality. Here are a few examples: If I were handsome [I am NOT handsome], I would have many girlfriends. If it were Sunday [it is NOT Sunday], many people would be in church today. I demand that Mona study more. [Not: "studieS." It is only my demand; it does not reflect reality, as in "Mona studieS hard very day.] Please remember: in some varieties of English, the subjunctive is not used so much as it is here in the United States.