Not sure about the filter on this GM trans, but indeed a flush is best. A case of 12 quarts should be enough for a relatively new car. But the total fill should be monitored at about 8-10 qts, while flushing.
The best ATF is Amsoil Universal Synthetic ATF. Easily exceeds ALL the ATF standards. Not only makes your car shift better, the lubricating qualities provide protection, total resistance to oxidation/thermal breakdown and cooler running that just plain will make your very expensive transmission last a long time. Doesn't make sense NOT to use synthetic ATF.
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