'If I were you' is the subjunctive mood and is used to express a hypothetical situation which cannot be true. The person speaking cannot physically 'be' the other person so you should use the subjunctive. That being said, there are millions of native speakers in the UK who ignore this rule and say 'if I was you/him/her etc.' I'm given to understand its usage is not as common in American English. I wonder how strict examination boards are here in the UK because it's something that's quite prevalent.

"I were" is correct. This is something that even stumps native speakers. I often hear this mistake haha

In singular and plural, "were"

I were