Series 3 Gearbox oil change top cover?

I'm doing a complete oil change on a series 3 gearbox so I know I need 1.5l , is it a good idea to put it directly in the top after removing the metal disc , I understand how topping up could be a problem and how the side fill would be beneficial in these circumstances because you don't know what's in there , however surely for a complete change you could use the top fill ?

you can pour it in the top but you must check the level with the filler plug on the side as you shouldn't overfill, it can cause hydraulic pressure and damage the box!