A man who spent the war in a Hokkaido plant tasked with offsetting Japan's petroleum shortages with man-made oil said he felt the nation's enemies were "incommensurably rich" in the key resource.

The government built a plant to make coal oil in Takikawa, Hokkaido, in 1941. Shigeo Kokubun was hired the following year and ran the coke furnaces where coal was burned to generate gas, which was then liquefied into oil.

Since the technology had been developed by Germany, four German engineers were stationed at the plant, along with elite Japanese engineering graduates from top-notch universities, he said.