The Magic Reappearing Carpet Stains! ...Any Advice?

My boyfriend and I have lived in our apartment for 4 months now, and when we moved in... the carpet was freshly cleaned and looked great. Our landlord stated that we would have to pay 325 bucks (I know, crazy right?) for a carpet cleaning if there's any large, horrid stains. ...Well, whomever lived in this place before us tracked in a ton of black oil and since we've been walking all over the carpeting, the entranceway looks like a dalmatian rug now! There's large spill stains in the oddest places too. I've tried everything to get them out! ...Does anyone have any ideas to remove the stains permanently, or should I just wait until the last minute and steam clean it myself?