The International Energy Agency said it is changing the way it calculates China's oil demand but acknowledged even this new method won't be accurate because of shortcomings in official Chinese data.

"Few oil-market issues are more critical yet more elusive than getting Chinese demand right," the industrial countries' energy watchdog said in its latest monthly report, published Wednesday. But it said assessing current and past Chinese oil use remains more art than science.

China, the world's second-largest consumer of oil behind the U.S., has accounted for roughly 10% of the world's oil consumption and 40% of global growth in oil use in recent years, it said. In 2013, the country's estimated oil product demand will be 9.98 million barrels a day, up from 9.96 million barrels a day in 2012.

In the past, the IEA, like many others, calculated Chinese "apparent demand"—a proxy for consumption—as the sum of Chinese refinery output and net product imports, it said.

However, it is unclear whether data from China's National Bureau of Statistics on Chinese refinery output include only state-owned refineries or also captured some independent refineries commonly referred to as "teapots," which are estimated to account for as much as a third of Chinese nominal refinery capacity.

Additionally, the absence of reliable data on Chinese refined-oil stocks is an obvious shortcoming, the IEA said. While China has produced some data on oil stocks since 2006, the numbers in recent years have been limited to percentage changes, and haven't included volumes, it said.

It also is unclear whether the published inventory data include oil products held by the smaller two of China's four main refining companies or stocks held by independent refiners, it said.

To better reflect Chinese demand, its new methodology will capture reported stock changes, it said.

Even so, percentage stock changes reported by the official Xinhua news agency only include inventories of gasoline, gasoil and jet-kerosene. They don't account for any changes in liquefied petroleum gas, naphtha or fuel oil stocks.

Despite the shortcomings in official data from China, other mainstream energy organizations include Chinese forecasts in their reports.

Wednesday, for example, the U.S. Energy Information Administration predicted China's oil demand would grow 4.4% this year. The IEA has forecast a growth rate of 4.0%.