About 20 percent of lead-free auto fuels in the Chinese market are not up to the required standard, according to a recent sample survey conducted by the State Quality and Quarantine Administration, Beijing Daily reported on Monday.

The result, published on Sunday, showed that only 80.6 percent of the No. 93 lead-free fuels, the most popular fuel grade in China, meets the required standard, while the figure stood at 78.6 percent for No. 90 grade fuels.

The survey also said that the quality of lead-free gasoline at the oil-refueling stations run by China National Petroleum Corporation (CNPC), and China National Petrochemical Corporation (Sinopec), two giant state-owned oil companies, were the best, with 93.9 percent meeting the standard.

High contents of formaldehyde and sulfur were blamed for the low quality of fuels.