ok heres why-
leaving in blue D is not good unless youre on the highway/freeway. leaving it in blue D causes the gearbox to constantly switch between 3rd and 4th gear when accelerating and this is not good for the box. 4th gear also requires more power to get back up to 80 when you drop your speed, white D does not have this problem, mainly because the gear ratio is 1:1 in 3rd gear.
as for those people that think fuel economy is worse in white D, youre wrong. using a calais dash showing instantaneous fuel usage, white d provides the same, if not better economy than blue when travelling around 80km/h