I think saying it's the "called for Allison ATF" is an overstatement, and unlikely to do much for this issue. According to Wikipedia, there are over 100,000 of these trucks running around with their transmissions filled with engine oil, and they are doing just fine. Unrelated to this issue, sure, switch to ATF if you want - I noticed some minor shifting improvement when I switched.
Can you explain the symptoms more? Does it sound like the engine is working hard but the transmission isn't converting it to movement? Is the engine bogging down until it gets some speed?
Normally, the torque produced in second gear (the first gear in use during normal operation) is ridiculously high. The truck can pretty much idle up the side of a building. There is a fairly large ratio jump between the 2nd and 3rd gears (first and second, in normal use), and mine tends to get revved up pretty good before it makes that switch, at which time it takes off in a quick increase in speed.
Without more info, some thoughts...
- Something with the turbo? Turbo have too much drag and it's not spooling up, creating more of a blockage than a benefit?
- Brake(s) stuck on? Do they get hot?
- Transmission stuck in a higher gear?
I'm just grasping at straws.
