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.
Originally Korea was a ruled by kingship, then Japan annexed Korea from 1919 to 1945. After Japanese surrender in 1945, the country was occupied by former Soviet Union and the US. There was discussion among winners of WWII about what to do about Korea.