grammar - Difference between 'If ..was' and 'If ...were'

Hi,

I have seen different usage of the phrase 'If..was' and 'If..were'. But I find it difficult to understand when to use which.

e.g. If I were the President of the United State, I would get the hell out of Iraq.

This looks correct. Can I use 'If I was' here?

Is there a rule?