Oil is an essential lubricant in your engine. It lets metal press against metal without damage. For example, it lubricates the pistons as they move up and down in the cylinders.
1 By the time of the Japanese surrender in August 1945, Korea was the second-most industrialized nation in Asia after Japan itself.  But the wartime mobilization of 1937-45 had reintroduced harsh measures to Japanese colonial rule, as Koreans were forced to work in Japanese factories and were sent as soldiers to the front.
The Empire of Japan formally annexed the Korean Peninsula in 1910. It had actually run the country through puppet emperors since its 1895 victory in the First Sino-Japanese War. Thus, from 1910 until 1945, Korea was a Japanese colony.