is this a single pipe or 2 pipe system
Thank you...you have a vacuum leak on the line somewhere...
Either tank fitting, oil filter, flare nut, at pump etc...
The longer the unit sits it loses prime and sucks air into the system. Start at one end and check every fitting you possibly can.
To verify you can have an oil tech run a vacuum test on the line.
You wont see a leak because it is sucking air in. The breach will draw air instead of leak oil.
Correct...if it were pump you would not have nice full lengths of liquid...it would be frothy and regular....the symptom you describe is the pump pulls the vacuum to pull oil from tank....then it fires and runs fine....shuts off...and then the vacuum is compromised and slowly sucks an air pocket in. Then on next startup it faults.
You will notice the longer it sets the more air is in line.
I see your thoughts....you just have to imagine having a bottle of liquid under a vacuum...if you poke a hole in it it will draw air until it equalizes...but until it equalizes it will still hold the liquid.