In my experience, the dirt from a transmission is fine powder, I'm
in favor of dropping pan, and scrubbing away dirt, + new filter. If you
want to be a real shark, change again a couple thousand miles later.
A motor lets you drain almost all oil out, not so with trans, which
only allows removal of a couple quarts, approx 2 gallons stays in the
internal passages. I recommend Mobil 1 synthetic trans fluid for hot
climate, severe duty, takes a few change outs to be * almost * all
synthetic. Conventional or synthetic, fluid should be changed when dirty.