You can sell the excess oil in your tank...it really is no different than playing the stock market.
I actually learned this from my Grandfather as a farmer. We always put up a stock pile of hay in the fall, then carefully figured out how much we would need for the rest of the winter and sell off some of it as the winter progressed. It gave us a little cash flow and ensured the price was the best for the given time period. Today I do that with firewood too. I sell my excess off at this time of year when the price is higher, people are running out, and the firewood dealers are all out of dry firewood. I don't gouge people, I actually sell it cheaper than those around me, and I am sure to get a quick, cash sale. This weekend I actually have some firewood that I am going to sell from my Grandmother's home. Its really no good because it has been drying for 20 years in her woodshed, but with people needing dry wood...even if it is too dry...now is the time to sell and make a bit of money I think.
With hay or firewood, it is best to keep careful numbers so that you know just how much you need, but if you can easily replenish your hay/firewood stash in the summer or fall, then there is no need to retain wood for next year. It is actually very sound practice...and lucrative.
If you have a tank full of heating oil and know you can fill it in the summer when it is a lot cheaper...why not make a quick buck on it?