The question seems to be, what constitutes "residual oil"? That remaining after the crew have activated the switch? Was the point of switch activation considered the "critical entry condition"? Questions, but no answers.
Brian, I take your point. To my mind residual oil is what is left clinging to gears, bearings, shafts etc and since there is no mention of discontinuing the drain, it would seem to be the intention to drain all the free oil out. But as you say, that is just my interpretation and demonstrates the difficulty of using the written word to communicate unambiguously (something lawyers have been cashing in on since the law was invented!).
HC