Drain the sump and refill with clean fluid.
Add an extra 1.5L of fluid.
Remove cooler hose at radiator. (you want to pull the oil return to gearbox hose)
Get a few 2L milk bottles for draining and put the hose in one of them.
Start the engine and run until the milk bottle is three quarters full then stop. (won't take long, 10 seconds or there abouts. DO NOT RUN THE BOX DRY!!)
Fill in another 1.5L and do it all again start the engine again.
Keep doing this until you've gone through at least 12L of fluid.
Reconnect the oil hose and top up the box as required.
This will replace all of the fluid in the box, not just the sump.
