I think you should follow the factory maintenance procedure on something
like a transmission.
If it's an automatic transmission, I shy away from flushes. I have a
Honda for which the factory procedure involves draining and refilling
the thing four times in succession, driving around a bit in between to
cycle the new oil in. This gets a good bit of the old oil out.
I have a Lexus that some corner gas station did a couple of transmission
flushes in; 10K miles later, the entire transmission needed rebuilt.
The previous owner did all this, and I inherited (quite literally) the
problem. That's why I shy away from flushes.
It seems to me that modern automatic transmissions are incredibly
complex, perhaps too much so, and that shade-tree mechanic concepts
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