I drain mine as I stated because that way I can get 8 litres into the car with no problem, others say they get the overfill signal, not me I can tip 8 litres in without stopping.

The reason they cannot is because some of the old oil is caught in the engine in various places.
These engines are all the same and will take the full amount of oil if drained properly, so I do a good drain. Not like some shops who pull the plug or suck it out and immediately start adding the fresh oil when the oil flows to a trickle, it is amazing how much oil will come out after it has slowed down to a trickle.