China's Number 1 Auto Factory, with its smokestacks belching clouds of black soot high above victorian street lamps and dingy brick walls, will turn out 60,000 shiny new trucks here this year, all with the unmistakable styling of the late 1930s.

In other aging factories nearby, built by the Japanese conquerors 50 years ago, the Chinese produce synthetic rubber, cigarettes, liquefied coal, machine tools and weapons on a sprawling industrial base that, to a recent visitor, appears not to have changed markedly since the Japanese left it behind at the end of World War II.

Today, however, the Japanese have returned in force to their former colonial empire here in northeastern China. This time, though, the goose-stepping soldiers and harsh military rulers of the past have been replaced by an army of well-mannered Japanese businessmen, bankers and engineers. They have come armed, at Peking's behest, with sophisticated machinery, blueprints to build new factories or remodel old ones, and millions upon millions of dollars in financing.

Although it may be well into the 1990s before it pays off, the Japanese view their new foothold in Manchuria as a key opening to China's slowly budding economy and one that could play an important role in helping to advance the country's ambitious plans for modernization.

In 1934, Changchun became the hub of a huge industrialization program when the invading Japanese established a capital here and installed Henry Pu Yi in an ornate palace as figurehead emperor of a puppet state they named Manchukuo. In its heyday, the colony had a Japanese population of 200,000, including some of the best and brightest of Japan's youth, who flocked here to oversee the building of railroads, dams and deep-water ports in an area that remains China's biggest industrial center.

Today, Changchun's broad, tree-lined avenues, which were patterned by Japanese engineers after those of Washington, are filled with Toyotas and Datsuns, a sign of Japan's more recent commercial inroads. Before the war, the Japanese university here graduated a number of individuals who still hold top-ranking positions in business and politics in Japan. Now, renamed the Jilin Polytechnical Institute, it trains Chinese technicians for industry throughout China.

Despite some of the positive aspects of their legacy here, however, the Japanese are still regarded with mixed emotions. Yang Qingxiang, editor of the Changchun Times, who was a teen-ager during the war, said, "We all had bitter experiences of the cruelty of the Japanese militarists." Echoing the pragmatic approach the Chinese have adopted toward their former rulers, he added, "The past is the past. The prospects for Sino-Japanese relations is now bright as long, of course, as they remain beneficial to both sides."

To accentuate the concrete benefits, the Japanese have extended more than $2 billion in government-sponsored, low-interest loans to China since 1978. Nearly three-quarters of these funds have been set aside for work on a massive petrochemical complex at Daqing and for oil exploration and development in the Bo Hai Gulf, both inside Japan's former colonial sphere.

Tokyo is now reportedly negotiating a second, larger package of official loans that will, like the others, be tied to the purchase of Japanese goods and services. This is in sharp contrast to the United States, which, as a matter of policy, does not extend direct economic aid to China because of a longstanding prohibition against direct bilateral aid to communist countries.

An administration request to have China taken off the list of countries that may not receive aid has been approved by House and Senate committees and is in the foreign aid bill now before Congress, a State Department official in Washington said. He added, however, that there are no plans now to request any direct aid for China. In the 1930s, Japan's bold land grab in Manchuria, which ultimately led to Pearl Harbor and war with the Allied powers, was aimed at obtaining vital raw materials and elbow room for the crowded, resouce-poor Japanese islands. While the old political motivations are gone, Manchuria's storehouse of mineral wealth still exerts a powerful hold on Japanese interests in the economic sphere.

The three provinces of the Northeast, a name the Chinese prefer to Manchuria, today provide China with two-thirds of its crude oil, half of its timber products and a quarter of its coal. In a trade-off envisioned by both sides, Japan would export increasingly larger amounts of industrial goods to China in return for progressively larger amounts of resources, particularly oil.

At present, Japan remains heavily dependent on the Middle East as a source of oil and buys less than 5 percent of its total yearly supplies from China. But China, in recent years, has loomed increasingly large in Japan's plans to diversify its future energy imports.

According to an article appearing in China's official Peking Review in April, "China and Japan share mutually complementary conditions by which we can clearly benefit each other. Japan needs China's energy and natural resources and markets. China needs cooperation with Japan in . . . technology, capital investment and managerial methods."

In one typical deal, the Japan National Oil Corp. and a consortium of 47 private Japanese companies already have invested $210 million in joint oil development in the Bo Hai Gulf. Late last year, Tokyo's offer of an additional $400 million in government-sponsored loans sparked a controversy in Washington among U.S. officials who speculated that Japan may have been using the funds to gain a favored position among foreign competitors in the bidding for potentially larger offshore oil reserves in the South China Sea near Canton.

The Japanese flatly have denied charges that they have tried to parlay their stake in Manchuria into broader economic concessions from the Chinese. Officials point out that while Japan's $9 billion-a-year trade with China now outstrips that of any western country, including the United States, it slumped sharply last year as a shortage of foreign exchange forced the Chinese leadership to cancel or drastically cut back key industrial projects.

According to Akio Suzuki, who heads the research department at the quasi-governmental Japan-China Association on Economy and Trade in Tokyo, the Chinese cash crunch has been a blessing in disguise. It has forced Peking, he said, to concentrate on the less costly refurbishing of old industrial sites such as the huge Anshan iron and steel complex south of Changchun that the Japanese built in the 1930s. As a result, "Japanese trading companies have been able to extend their networks on a broader basis" throughout China, he said.

"You get the feeling," said one American expert on China's economy, who has made frequent visits to Manchuria, "that the Chinese would like to deal with more Americans and Europeans, but the Japanese are already there and they've got the technology and the money with them."

While the messianic zeal that originally brought the Japanese to Manchuria has largely disappeared among Japan's affluent, western- oriented postwar generation, some older Japanese still see China with a sense of mission.

Kiyoshi Morikawa, who helped pioneer the development of the huge coal mines near Changchun under colonial rule, said, "We Japanese launched an invasion that caused the Chinese people a lot of trouble. Now we have a guilty conscience and want to cure it by helping the Chinese modernize."

Today, Morikawa, an executive at a large chemical company in Tokyo, belongs to a group of influential Japanese businessmen who are using their old Manchurian connections to provide China with the latest Japanese technology.

"The population of China is now more than 1 billion," he said, "and if they starve, peace in Asia will be impossible to achieve."

Reflecting the more businesslike approach of most Japanese, however, one leading China-watcher in Tokyo said, "Japan is not going to rely on China as a major supplier of resources anytime soon but, historically and geographically, our two countries are very close. If we can help them exploit their markets economically, then why not?"