I just siphone out 4 liters and put back in 4 liters of new. Flushing is not always favorable because it tends to dislodge dirt.
Dealers always recommend a flush but they want to make money and rob your wallet no matter how reputable they are.
siphoned out? did you run a hose into the fill port under/next to the battery??
As for the flush I've read it's a bad idea as well. I feel like a couple drain and fills every 10,000 miles accomplishes the same thing as a flush without forcing all the crud all around all the sensitive tranny parts.
anyway I just did a drain and fill at 79000 miles, first time for this used car I bought. people think the only maintenance is oil changes but with this car it's tranny, differential, transfer case, spark plugs, brake fluid, coolant fluid, power steering fluid and definitely something more!