From: Kila Hana
Interesting. I could be totally wrong but I thought they discovered during actual war time usage, that only had enough oil for two months.
By March 1941, the Japanese had managed to squirrel away about 42.7 million barrels of oil, most of it imported from California and Tarakan in Borneo. This was held in some 7,000 oil storage tanks, also purchased from the United States. IJN petroleum reserves at the outbreak of the war amounted to 1,435,000 tons of crude oil, 3,634,000 tons of of bunker fuel, and 473,000 tons of aviation gasoline. This was thought to be adequate for the first year of war, but consumption greatly exceeded prewar projections. The Army estimated it would require 5.7 million barrels of oil per year while Navy requirements were estimated at 17.6 million barrels per year and civilian requirements at 12.6 million barrels per year. This ended up being a considerable underestimate in the first two years of the war.
The Japanese by all accounts badly bungled their limited resources of oil. They took what seemed like the logical step at the time of trying to establish a synthetic oil industry based on their sizable supplies of coal, but this effort failed because of a lack of technical expertise and shortages of alloying and catalytic metals for the synthetic oil plants. The Japanese also suffered a serious loss when an American submarine by blind luck, chanced upon, and sank the one ship carrying most of Japan's few oil industry technicians to the recently captured oil regions. However, demolition of the oil fields of the Netherlands East Indies was carried out rather poorly and production rebounded quickly. The real problem was getting the oil back to Japan. The Japanese tanker fleet was never adequate, and insufficient priority was given to building more tankers. So much of the production from Southeast Asia never actually made it to Japan.
Although production in Japanese-controlled areas peaked at almost four million barrels a month in 1943, imports to Japan never exceeded about 1.4 million barrels a month. In the last two years of the war, the Japanese Fleet lost its advantage of interior lines of communication because of the necessity of basing much of the fleet near its fuel sources, at Singapore or Tawi Tawi.