2005 L322 4.4 V8 gearbox oil change

OK just got new car and it feels really good and I'd like it to stay that way.

2005 (54) Vogue

51k miles full dealer history - 3 services completed.

I'm looking at changing the transmission oil with type LT 71141.

I can get 12l or 20l - I understand you need to do a few changes to get as much of the old out as possible, so will 20l do 2 flushes and a third to stay in?

Whats the best way to go about this, regarding not having to replace filters and seals, etc every time?

I've read a fair but about doing the job, engine running, change through D P R, etc.

But not seen much about the flushing procedure.

Many thanks.

Drain oil, remove sump, change filter, replace sump refill oil, drive for a few miles, drain oil and refill.

You will use approximately 6 or 7 litres each time you drain

or you can do the generic method ie,remove oil cooler comnection and place in bucket, start engine and let it pump it out,if you have a dipstick filler then keep topping up the gearbox untill the new fluid is through.this way you get it all in one sitting.if you do not have a dipstick filler,then you stop the engine, refill the sump and repeat the first op untill it new fluid is through.