WM-300
Joe Donnelly wrote an article in last The Register (I think) about changing the fluid on his 04 NV5600 at 5,000 miles. His experience was that changing the fluid smoothed up shifting.
I found the same on my 01. 5 HO 6spd. When I went to change the fluid at something like 25,000 miles, the oil started running back out of the hole before I was close to putting the amount of fluid spec'd in Service Manual back in. Ended up jacking the truck way up to allow the rest of the fluid to go in. Substantially smoothed up shifting. Probably put 1. 5 pints more in, after jacking it up. Never hurt anything. Shifted great till I sold it with 34,000 miles.
My 04. 5 has 3900 miles on it, and when it gets to 5K I am going to change it also. I am also planning on putting one of the filter kits on from Genos when I do it. I know it is supposed to be a lifetime never change fluid, but it makes a difference.