Garry,
There's a seasonality to it, too. Some years ago #2 fuel oil (which is heating oil, essentially diesel w/o the additives) regularly was cheaper in the summer and more costly in the winter. I had a business in Vermont, USA that used a LOT of heating oil; it was a large building. We pre-bought the fuel in July from the local dealer at the summer prices (usually 2,000 gallons or more) and then used it all year until the next summer. We locked in our price, and he either locked in his price or actually bought the fuel and stored it for us.
Today the fluctuations in pricing are somewhat random. You see what I call excuses for price increases: insurgency in Nigeria, prices up. OPEC meeting, prices up. "Changeover" to summer blend + the inevitable midwest refinery fire or pipeline burst, prices up. Shortage of additives, prices up. My neighbors own an oil distribution business, supplying gas stations and marinas and industrial facilities with gasoline and diesel. They used to store hundreds of thousands of gallons of both in their tanks--a week or more--but the fluctuations have turned the business into a casino operation, so no more than a couple of days supply is stored now. Even with this, they often get caught with gasoline that is retailing for less than they paid wholesale for, because of price fluctuations.
Michigan is also a bit different I think, with the disparity between diesel and gasoline. In many other states it isn't as drastic.
