Remove the rocker arm assembly from the head and remove the pushrods, keeping track of where they came from so you can put them back in the same place. Don't knock any of the valve caps down into the engine! The oil comes up through a hole in the rocker assembly rear bolt post so, remove the spark plugs so the engine can rotate quicker and then crank the engine with the starter. Oil should come out of the hole next to the rear rocker assembly bolt hole. If you get oil, then the problem is in the rocker assembly itself. If no oil, there is a blockage somewhere down below.