Check your delivery ticket to ensure that the amount of gallons delivered was mechanically printed on the ticket. Many states do not allow the gallonage amount to be handwritten, it must be mechanically printed on the ticket. Also, be sure to retain all your delivery slips and check to make sure the price per gallon appears on the ticket at time of delivery.
By sending in extra money each month or making an extra payment at the end of the year, you can accelerate the process of paying off the loan. When you send extra money, be sure to indicate that the excess payment is to be applied to the principal. Most lenders allow loan prepayment, though you may have to pay a prepayment penalty to do so. Ask your lender for details.