Let me see if I can be more clear. I would say "If I were a rich man, I wouldn't be living in this apartment," because I'm not a rich man. To my mind, there's not exactly a difference in meaning to say "If I was a rich man, I wouldn't be living in this apartment" so much as there's no meaning because to use "was" in that sentence doesn't make sense. The only time I'd use "If I was" would be in a different kind of sentence, like "If I was rude, I apologize profusely." Or maybe, "If I was a rich man, I sure don't remember it."
The difference in usage to me seems to be that "was" is talking about the past, whether or not there is any probability involved, whereas "were" is talking about some hypothetical present.