Douglas,
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?