From: Connecticut, USA
You see it when you click on a base, under Oil Storage and Resource Storage. The manual is vague on this, but I believe it works like this: Oil and Resources gets created by oil and resource centers. This gets converted into fuel and supplies, I think with your heavy industry. So some bases, like the US West Coast, will produce an ongoing stream of fuel and supplies due the fact it has all it needs - the raw materials and industry to convert it, at hand.
Japan is different. Even though they have large oil, fuel, resource, and supply stockpiles, it is a finite amount. They have the industry to convert fuel and resources, but not fuel and resource centers. So these need to be captured, hence the historical reasons for the war to begin with. You can capture the centers, as well as bases containing oil and resource storage, along with finished supply and fuel. The supply and fuel can be used where needed, while the resources and oil can be transported back to Japan via auto-convoy.
You can also disrupt the production system by aerial bombing industry in the USA, Hawaii, Australia, or Japan. Much of the production system, of which auto-convoy is a part, goes on behind the scenes and does not need to be managed by the player.