Great concept but flawed execution.  It would literally take hours to remove all 7 liters of oil from my BMW 3.0L I6 engine.  It doesn't matter if the oil is 70F or 210F.  The extraction is SLOW.  That's assuming the vacuum remained constant.  It doesn't.  I have to go back to the extractor and pump some more every 10 minutes or so.  Otherwise the flow of oil will stop.