Looking stricktly at the difference in the volume of oil in the reservior, you would want to change the oil when the most oil is able to drain from the resevior. With double acting cylinders, the volume of oil in the barrel of the cyliner is less when the rod is retracted. The difference is the volume of oil that the rod has displaced by being inside the barrel. There is still a good deal of oil in the barrel when the rod is retracted. That all being said, it isn't much of a difference in volume so do whatever is easiest for you.