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.
Korea was split in half after World War II. Japan ruled over Korea from 1905 until the end of World War II, after which the Soviet Union occupied the northern half of the peninsula and the United States occupied the south. Originally, they intended to keep Korea together as one country.