This one’s easy….. Allison requires it at 6000 miles. Then you’re off the hook until many miles later. (30,000). Here it is:
You’ll find it on the bottom of the transmission, can’t miss it. You’ll need a little Allison Fleetrite Synthetic ATF to top it off again when you’re done. I bought a gallon, some people bring a small container and schmooze the shop out of a few ounces out of the bulk barrel.