Would you mind if I opened the window?
This is the most grammatically correct usage. You should use this.
Would you mind if I open the window?
This, while technically incorrect, is still used a lot by native speakers.
Your other two sentences are in a similar situation:
If I were you
This is correct. While normally we use was in the singular case, for hypotheticals we use were.
If I was you
This, while technically wrong, is still used all the time by native speakers who don't understand the nuance of the subjunctive mood.