Well, Tom........as you probably assumed long ago (I see this thread is from 2008) your old realtor was either asleep for 25 years.....or she didn't get around much.
The concept of paying for oil in the tank is ANCIENT. All items like this are TOTALLY commonplace in real estate law, and in fact there's a document that is part of the closing that was created JUST FOR THIS.
It's called the STATEMENT of ADJUSTMENTS.....which is exactly what it sounds like.
Hope you fired that agent when she told you that!