As far as I know, to do a proper fluid change you have to do a combo of both. Removing the sump plug does not drain all of the oil, so you drain it first from the sump plug, refill with the same quantity that was drained. You then move on to the cooler pipe, turn the engine over and pump out fluid. re-top up with the amount removed, and keep repeating at the cooler until you start getting clean oil out of the cooler.
Thanks, Could you tell me where I could find the gearbox oil cooler pipe please?
Thanks
Arif