China's diesel consumption shrank 1.2% year over year between January and July this year. Here, a driver fills diesel into oil containers at the back of his truck at a gas station in Suining, Sichuan province on December 21, 2011 . Reuters

Oil bulls have swapped friends in China, parting with the country's industrial complex in favor of its burgeoning class of car drivers. They are going to discover that these new pals aren't as dependable.

China's economy may be rebalancing away from investment toward consumption, and its petroleum use is starting to reflect that. Global energy investors can't afford to ignore these changes, given that China accounts for 40% of the world's oil-demand growth.

On the industrial side, the slowdown has begun. Because industrial inputs such as diesel, fuel oil and petrochemicals constitute 70% of this market, China's oil demand was mostly flat between January and July compared with a year earlier. Overall demand might spike later this year as Beijing releases stimulus in small doses, but the broader trend will be hard to change.

Take diesel, China's most widely used fuel: Its consumption shrank 1.2% year over year between January and July, according to consultants at Energy Aspects. Partly, this reflects the trucks that use diesel hauling less heavy industrial freight such as coal.

But China's expanding rail network is taking market share, too. Trucking itself has started adopting natural gas, which will displace 4% of road diesel demand this year and 10% by 2020, says Wood Mackenzie, a consultancy.

China's saving grace for oil now is gasoline. Demand soared 10.3% year over year during the first seven months of 2014, in step with consumers buying roughly 11% more cars during that period.

Most analysts expect Chinese car sales to keep growing, but that doesn't mean gasoline will keep up the pace. Earlier this year, regulators proposed strict fuel-efficiency standards starting in 2016. If approved, car makers will have to reduce fuel consumption by 6.2% on average every year between 2016 and 2020.

More generally, consumers demand nowhere near as much oil as industrial users do. For every 1% increase in gross domestic product, oil demand today increases by about 0.3%, says HSBC. That is down from 1.5% a decade ago, when China's industrial expansion was at full throttle.

Refiners outside China are already feeling this shift. China built too much capacity to process crude oil, including into diesel, says Citigroup's Ed Morse. Now it has begun to export that excess capacity much the way it floods the world with cheap steel. China has exported a net five times as much diesel these last seven months as it did all of last year, according to data provider CEIC.

This has helped push down refining margins across Asia, partly why shares in South Korean refiners SK Innovation 096770.SE -1.78% SK Innovation Co. Ltd. S. Korea: KRX KRW93,800 -1,700 -1.78% Aug. 25, 2014 3:00 pm Volume (Delayed 20m) : 302,215 P/E Ratio 34.32 Market Cap KRW8,830.46 Billion Dividend Yield 3.41% Rev. per Employee KRW29,131,800,000 9800096000940009200010a11a12p1p2p 08/24/14 Heard on the Street: Drivers W... More quote details and news and S-Oil 010950.SE +0.75% S-Oil Corp. S. Korea: KRX KRW47,000 +350 +0.75% Aug. 25, 2014 3:00 pm Volume (Delayed 20m) : 442,875 P/E Ratio 21.19 Market Cap KRW5,251.99 Billion Dividend Yield 0.64% Rev. per Employee KRW11,937,200,000 48000470004600010a11a12p1p2p 08/24/14 Heard on the Street: Drivers W... 07/02/14 Hanjin Energy Approves Plan to... More quote details and news have suffered this year. While China's major oil companies have pledged to trim investment in refining, it may take time for capacity to conform to the new reality.

China's economic emergence drove oil prices for much of the past decade. With consumers now in the driver's seat, investors may have to get used to a lighter foot on China's pedal.

Write to Abheek Bhattacharya at