Thank you guys, it's helping.

As Ruthi says, the subjunctive is used here. It is used in sentences with the construction 'if [pronoun] [subjunctive] [something], [pronoun] [conditional] [hypothesis not fact]'. (In conditional sentences where hypothesis is not a fact.) Other examples: If he were here I would tell him what I think of him. If I were you I would stop now. Were this true, it would be a serious matter. Were they to have won, they would have been very rich. (past) Were they to win, they would be very rich. (past/present) If they were to win, they would become very rich. (future)

I think both options are correct, but people usually use "If I were You". My english teacher explained me something about that some months ago but I only remember I can use "If I were You" instead of "If I was You" ;) It's likely only be an exception. https://www.youtube.com/watch?v=ZpF2AvNWOH0

We use the past simple plural form for an imaginary, hypothetical situation. The reason is that this is called "subjunctive" to indicate something that is contrary to fact. Many Native speakers acually say this incorrectly, so they will say "was" instead of "were".