20k Need ATF Flush?

[Jetcool]

Was thinking of doing an ATF flush for my under 1 yr old FLG9's 20k servicing. Any comments whether this is necessary, or just a simple drain-and-fill will suffice? Was thinking of cleaning up the metal bits from the car's running-in.

 

I was told that i may need to use up to 18 litres of ATF fluid to do an ATF flush, is this info accurate? Thks for yr help