The gearboxes will need 2.4 litres from empty,do not fill the box using the overflow method at the level hole,just drain it and refill with 2.4 litres and then refit the level plug.
Personally I use Total BV Transmission oil in PK6 van gearboxes,the boxes need a light flushing out with a little brake cleaner before refilling.Many people will argue that it's the 'wrong' oil,but it's what the nice man from Total Technical told us to use a few years ago,and I have personally seen a PK6 box last well over 200,000 miles using this oil (the box had already covered 100,000 miles on the original oil from the factory which was changed as part of a clutch slave cylinder replacement for the Total BV when refitted).