Car oil is something that can be very expensive, especially if it is bought from the wrong place. If you have to buy oil from a motorway service station for example, you may be paying up to twenty pounds per litre compared to six or seven pounds elsewhere.
Often people have to buy oil from motorway stations or forecourts regardless of the price because they have completely ran out and cannot risk driving it any further. A simple check every few weeks or a month will give you enough time to know when it needs topping up allowing you time to shop around for the best price.
I recently had to buy some oil and found that surprisingly the cheapest place to get it was my local garage. They were charging nearly half as much for the oil as some of the main supermarkets and the quality was just as good.