Assumes a relatively free market.
A very basic commodity whose pricing is in the hands of a very few is an exception. There are choke points. OPEC, for one.
Once oil gets past OPEC, another choke point is in the retail distribution. A very few people can make decisions which affect the entire retail market, at least in the US.
Collusion would be difficult to prove, and is impossible to ignore.
A parallel is airline ticket prices. They move together. The variation is in what they will do to fill seats not sold at a higher price.
Arf