China has for the first time begun to offer its oil for sale in the American market, another of several recent signs that that country has decided to develop its unmeasured oil reserves.

The Chinese recently approached Gulf Oil Corp. through an intermediary, offering crude for sale, according to sources within the industry.

In addition, in the last few weeks the Chinese traded oil to an American auto company, presumably in payment for some vehicles. The auto company has since offered the oil to refiners, according to the authoritative Petroleum Intelligence Weekly.

U.S. and British oil companies have also held talks with the Chinese government regarding more conventional oil sales to the West, according to the National Council for U.S.-China Trade.

Speculation about deals bringing oil from China to American refiners and, ultimately, to the American gas pump, began spreading through oil industry circles just weeks after Peking made public its decision to invite major American oil companies to invest in developing China's untapped oil reserves.

Asked about the prospects that China could become a significant supplier for the U.S. market, one international oil specialist at the Department of Energy said, "The amounts are so small they could be a swallow."

U.S. imports now average more than 8 million barrels a day, accounting for half of this nation's daily consumption.

China's oil exports, according to Central Intelligence Agency estimates, will run nearly 300,000 barrels of oil a day between now and 1982, rising to a half million barrels of oil a day by 1985.

National Council for U.S. China-Trade Vice President Stanley Young says the Chinese are looking to oil as a major source of financing for their own development, but warns, "It is better to be conservative rather than getting highly enthusiastic at this stage."

International oil experts are split on what China's oil potential may be. Estimates range from about the same as Alaska's North Slope, to a 70-billion-barrel projection that would be nearly twice U.S. proved and probable oil reserves.

Young, a former Exxon executive, says that major American oil companies, Phillips, Exxon, Penzoil and Union Oil, who are negotiating with the Chinese to explore for oil, wouldn't go "unless there were prospects."

Total U.S.-China trade, which amounted to about $375 million last year, is expected to rise to more than $700 million this year. Oil rigs and other drilling technology suited for developing China's untapped offshore fields have accounted for an increasing share of U.S. sales to the Peking government.

So far more than 95 percent of China's drilling activity has taken place onshore, much of it in the famous Taching oil fields.

One obstacle posed to Chinese oil's entry into American oil markets, especially on the West Coast, is that they have been depressed - at least temporarily - by the world oil glut.

China's exported oil also is high in wax content and heavier than oil from the Middle East, making it less economic to refine.

Much of China's oil exports are obligated under agreements Peking signed with Japan this Spring.

Undeterred, the Chinese have pressed on to develop their oil resources.

Last May, Chinese Communist Party Chairman Hua Kuo-feng committed the Peking government to developing the equivalent of 10 more oilfields the size of Taching, China's largest.

Last January a high-ranking delegation of 16 oil and gas experts toured the U.S. oil installations. Energy Secretary James R. Schlesinger Jr. is expected to visit China in October to discuss China's oil development plan's and energy research and development.