Allison has a fliud change schedule. For example, the factors which affect the service intervals are whether you have a retarder, transynd or dextron, or the type and size of transmission filter.

A coach without a retarder, using other than transynd, and having a 2" gold seal (not high capacity) filter should have its fluid and filters changed every year or 25,000 miles whichever comes first.

That jumps to 150,000 mile fluid changes, 75,000 mile filter changes (high capacity filter) or 48 months, whichever comes first if the coach has a retarder, or 300,000 if it has no retarder.

http://www.allisontransmission.com/s...voc=MH&type=4k

You can use the above site to determine your service intervals.

The retarder is clearly considered to be subjecting the transmission fluids to harsh duty.