The instructions for the form:
Use type "2":
Off-highway business use (for business use other than in a highway vehicle registered or required to be registered for highway use)
You must retain receipts for the fuel purchased. They need to state the number of gallons (what you paid is irrelevant to this), where you bought it, and when. You will need to separate receipts between on-road, and off-road use gasoline, because you cannot claim the credit on fuel that went into an on-road vehicle.