When should I flush my transmission
I would refer to the owner's manual for the suggested service interval for the transmission. I like to do mine every 30,000 miles.
Never. Just change the filter and fluid.
Flush machines now days are fluid exchange machines that have a bladder between the old and new fluid keeping it separate. "Flushing" on a machine like that will cause no issues as it is a fluid exchange machine. You will still not get all the old fluid out, but this is still better than drain and refill.
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