The normal formula [6× (1-1/4)+8] ⊙ [1-1/2× (1-kloc-0//4)] = 20.

Thinking: Do you understand the sentence "1/4 is used for the second oil storage" as-

"1/2 oil is used up again, and 1/4 6L oil is also used up."

At this time, there is still 3/8 oil +4.5 liters of oil, and the title says that adding 8 liters will be full.

Obviously (4.5 liters +8 liters) is equivalent to 5/8 of the oil.

(* _ _ *) Hee hee ...