The hydraulic oil is not used as a 'fuel' to burn but to control the actuators for the grid fins. So if you lose control because you run out of hydraulic oil the last bit of the descent will be uncontrolled. So they almost made it with enough hydraulic oil, which means that next time the grid fins will work all the way down, which leaves a ton of other possibilities for mishap (too high a descent rate, rogue waves, barge being slightly at an angle and so on). Makes you wonder why the hydraulic circuit isn't closed, probably they figured an open loop system is more reliable than a closed one (half the gear and no need for a sump, which comes in handy when you have to deal with gravity coming from all directions).