OP, it's normal for the oil to go down a little bit between oil changes. For example, when your oil is hot it's letting off vapor which then goes through your intake manifold, and into your combustion chamber to be burned.
Some manufacturers if not all have baffling in the valve cover(s) to catch the oil vapor so it drops back down into the sump and not go through the PCV system with the blow by gases to be burned.
For those that do not know, the PCV system replaced road draft tubes which just expelled blow by gases and oil vapor to the atmosphere.
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Ford Diesel and Heavy Line Technician at your service. 
Last edited by Dreadnaught; 06-29-2017 at 11:30 PM.
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