Before I start, it will help if you watch the following videos.
And here’s another from Eminem, featuring Bruno Mars, entitled “Lighters”:
If you notice, the songs I just mentioned (which I love, by the way) contained the phrase “if I was you” as part of the lyrics. Oh, and if you watch Rush Hour 2, there’s a part where Chris Tucker said this phrase to Jacky Chan, too (I love Rush Hour too so don’t get me wrong).
I wonder if this phrase is correct so I did some research and found out that public opinion is almost divided, even in the countries where English is the primary language.
According to BBC U.K. :
“If I was” is a colloquial alternative to “if I were”.
In speech, many English people say “If I was”, but “If I were” is held to be grammatically correct.
It is better, therefore, to stick to “If I were”, as in these two examples:
“If I were you, I’d fill the tank now so that we don’t run out of petrol.”
“If I were to call in on my way home from work, would that be all right?”
In my analysis, “if I was you” may be commonly used in colloquial English (slang) but in written English, it is a mark of poor grammar. “If I was you” is non-standard English and would therefore be considered more appropriate in informal contexts, whereas “If I were you” is Standard English and would therefore be considered more appropriate in formal contexts. Neither is more correct.
I’m not sure how many of us even noticed this the moment we heard the songs. Perhaps, that’s because of our perception. Some of us subconsciously believe that “if I was you” is correct or acceptable (at least in the slang-perspective) while some believe that “if I were” you is grammatically correct, and putting it on an informal context (such as song lyrics) won’t do any harm.
I personally believe that “if I were you” is grammatically correct and should be used, especially if the one who will deliver those words is a known figure and is about to influence the major population through any available media possible.
Of course we are free to use colloquial terms in our daily conversations or even in informal writings. However, we might want to prefer grammatical conventions especially when academics, business writing or any formality is involved.