If you are expressing a
conditional future, the correct expression to use is “if I were.” So it should
be “If I were you,” “if my father were here.” However, over the years, “if I
was” has become very popular and is gaining respectability. So “if I were” and
“if I was” are now used interchangeably in both Britain and America.
Now, this is my advice:
if you want to impress grammatical purists, use "if I were."
Otherwise, feel free to use "if I was." I personally, use "If I
were" in formal contexts and "if I was" in informal contexts.
© Farooq A. Kperogi