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 divided into a northern Soviet zone and a southern American zone after World War II, and Soviet resistance to reunification led to the establishment in 1948 of two separate countries, with the Korean War (1950-1953) leaving the peninsula divided along much the same line as before.