Is it just possible that there is some misunderstanding by what is meant by "flush"?
To me, it means draining as much of the old oil as will come out then filling with a thinner "flushing oil" (intended to penetrate all the little nooks and crannies and get the crud out), drain that and then fill with lovely fresh oil of the correct spec.
The above is of course in an engine, not an auto box, where the "character" (cleanliness and condition) of the oil will determine (to a certain extent) the performance of the box.
I can appreciate the reluctance of some to (potentially) dislodge crud and then to have that suspended in the oil until the umteenth part change cleans the oil enough to not impact the operation of the box.
Cheers
Dave
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