If you have a valve and oil flowing through it... then you shut off the valve... I assume that most of the oil will stop flowing and the pressure will basically be 0 at the  safety valve and atmosphere at the end of the pipe.
so if from left to right you had a shutoff valve then a 5' hor. run then 10' vert. rise then 10' hor. run then a 15' vert. drop and then a 15' hor. run then there is essentially open end pipe into a furnace oh and its 1/2" steel pipe.
Given enough time (probably a few minutes, depending on the oil viscosity) the 10' hor run, the 15' vert drop and the 15' hor run will drain into the furnace. Air will have to flow from the open end into the pipe to replace the volume of the drained oil. If the valve is shut tight no siphon can form.
The pressure upstream of the closed valve will be the supply pressure, plus or minus any changes in static head from the source of pressure.  The pressure downstream of the valve will be 10' of your oil.
The two important forces are gravity and surface tension.  If you take a drinking straw, with a diameter of about 1/8", fill it with water and hold it vertically with your finger closing the upper end the water will not run out.  Gravity is trying to pull the water out of the bottom of the drinking straw and surface tension is holding it in the pipe.  With the small diameter surface tension wins.

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