The only thing I would add is, place a gauge on the valve and close the line between the tank and piston.
A 6" diameter x 40 foot long piston would hold almost 60 gallons of oil. And you say you added 40 gallons in the last 3 months. I would believe you would likely see the oil if you hung or landed the car, disconnected the piston, lowered the piston and looked inside.
Or is this a 100' long piston?
Also maybe someone else may have seen this or know? Can a piston fill to a point where it could hold enough to keep additional oil from pushing in?
I have also seen what I call a flapper leak, the rust or corrosion is semi-sealing the leak most of the time and with full load it holds even better(until it doesn't). I have left cars like yours fully loaded after a relief test many weekends and not found any movement. Once I showed up on Monday morning and the car was in the exact same spot a few feet above the bottom landing(this test the piston joints as well) then we left to get a cup of coffee and call the boss. When we got back from coffee and talking to the property manager we were shocked to find the car on the springs and it wouldn't even pressurize the jack. When we pulled the cylinder out of the ground there was a rusted hole the size of a dime near the bottom of the jack. Scary Crap.