if it is coming out a bit slow, light a fire under the diff.
oh and a little trick i think i have discovered, if you want to change the gearbox oil in a sprinter (i will do this soon) take the centre console out, unbolt the gearstick and just pour it in where the gear selector lives, as far as i can tell this should drain into the gearbox (looks nice and gbox oily at least)
(until it runs out of the inspection plug that is, that you have taken the bolt off)
might be easier just to squirt it in the level hole, i think that is the better option actually.