Such wide swings have led some analysts to express doubt on the reliability of information from China. The reality is that it will take some time before anyone knows for sure what Chinese demand is this year.
There are other clouds on the horizon - among them fears of an American economic slowdown, last week's strong economic data notwithstanding, or an outbreak of avian flu, potentially reducing international air travel and hurting regional economies.
"The biggest worry I have for next year is geopolitical," Mr. Hofmeister of Royal Dutch/Shell's American unit said. These, he said, included "disruption in supplies, greater distress in the Middle East, a slowdown in China, a collapse of Iraq."
Mr. Hofmeister continued: "My other worry is a slowdown in the U.S. economy. When you combine the high energy prices, the difficulties with vehicle sales, the backup in the supply chain, the problems with the retail chain, you could end up with a slowdown in demand."
In addition to these questions about where consumption may be headed, there are also uncertainties over how quickly oil companies are adding supplies.
In a much-discussed report, a prominent consultant, Cambridge Energy Research Associates of Cambridge, Mass., estimated that global production would rise by 16 million barrels - or nearly 20 percent - by 2010, far outstripping the estimated growth in demand over that period.
Some analysts fault the report, saying it is far too optimistic. And oil companies have been criticized lately for investing too little in exploration and refining capacity. A few weeks ago, the Senate held hearings and summoned the heads of the top oil companies to testify about their record profits and to justify their investment decisions.
Saudi Arabia, the world's largest producer, has been assailed for investing too little in new capacity.
One of the problems, the industry argues, is that forecasting the level of supplies needed to keep up with long-term demand can be tricky.
The International Energy Agency, in its recent global outlook, laid out diverging visions of where energy demand might be headed over the next quarter-century, and it found that the difference between them was nearly 20 million barrels a day by 2030, or twice the current Saudi production.
"This is a new era," said Fatih Birol, chief economist at the energy agency, which is based in Paris. "The risk is less of a drop than higher prices in the future."
But not everyone is persuaded. Shell, for example, assumes the opposite, lower prices, when it looks at future projects.
"We've very conservative," Mr. Hofmeister said, adding that projects he considers must be profitable at $25 a barrel. Some analysts argue that the break-even point on oil is unreasonably low and that it shuts the door to additional, if more costly, supplies.
But for oil executives, their experiences of the 1985 and 1998 price collapses remain a stronger influence than their belief that the world has entered a period of more expensive energy.
"If demand slowed, we'd be in a world of cheap supplies again," Mr. Hofmeister said. "But decisions I make today must still make sense in 25 years."
As Lee R. Raymond, Exxon Mobil's chairman, said in his recent testimony to Senate committees, "In the energy industry, time is measured in decades."
He said Exxon was involved in a $13 billion project in eastern Siberia that began 10 years ago and was expected to produce for 40 years. "All told, that's more than 50 years for one project," he said. To drive home his argument, he added, "Fifty years ago, Dwight Eisenhower was president."
He might have added that this was before the nation's Interstate System of highways was completed and before S.U.V.'s became Americans' vehicles of choice.