There was a recent post about an Allison automatic that the fluid was black after 20,000 miles of driving. First off, trans fluid doesn't just change to black suddenly. What this tell me is that the owner had not been checking the trans fluid level, smelling the fluid for burnt smell (rub the oil off the dip stick with your fingers, rub it between your fingers and smell it. Rubbing you can feel if it is oily, and smelling for burning). I know that I drive about 5,000 miles a year with the bus, so 20,000 miles would be 4 years of use. I can't believe that anyone of us would let the trans go that long and not check it to make sure the oil is still in a reddish state of color. When I bought my bus, the trans (V730) was serviced and oil changed. I next changed it 12 years later, but ultimately was only about 20,000 miles later.
The smallish AT545 and MT640 transmissions work very hard and have smaller oil pans, should be changed every 12,000 miles. If you have a HT740 or B500 World, then 25,000 miles with regular trans oil is fine. With Castrol Transynd synthetic fluid, you can go 25,000 miles between filter changes and 150,000 miles between oil changes-but the fluid is about $60.00/gallon.
My point-please check the fluid levels and quality of the fluids often. These buses are made to run and run hard for years and years, but with the fluids being kept up at level, clean, and using quality products. Good Luck, TomC