OK, the puropose of them originally was to drain off kerosene that seeped passed the rings while running. You would drain it down to the lower petcock, the fill to the upper one with oil. You would do this in the mornings as the kerosene would seperate out overnight. Why they never changed them to just one ill never know. Even some of the straight gas tractors we had had 2 petcocks, so in short, fill it until it comes out of the top one.