Simp, I'm also curious but don't want to hi-jack the thread with a previous discussion I have read a bit on. CSM Davis tells people not to put Dexron trans oil in these transmissions, Allison recommends the Dexron ATF. I changed mine before the GA Rally and since it was clear coming out, I went back in with 15W40 Rotella. If it had been red dyed coming out, Dexron would have went back in. I noticed no change in shifting or temps which would make sense since I basically made no change other than putting new fluid of like kind in place of old and replaced the trans filters. The old ones were actually spotless, I cut them up looking for trouble and thankfully found nothing.
I don't wish to turn this into an oil debate either. Allison has a pdf called "mechanic tips" for the mtb 600 series of trans. There is an oil spec chart on page 5 that shows the C4 rating approved fluids. Basically, you choose what weight fluid depending on your use of the vehicle(on road or off road) and the ambient temperature at which the vehicle operates in. On page 6, It shows the different fluids and their prospective minimum temperature limit, meaning you need to preheat your trans oil with an electric glue on heater on the pan, or run the truck 20-30 minutes in neutral at idle to warm and thin the transmission fluid.
I think what causes these transmission to fail with 15-40w motor oil in them is not letting the oil warm up and thin out before driving, especially under a load when accelerating. Since I live in a southern climate, I chose Orielly's premium (green bucket) trans hydraulic oil, part # 74209. It is a 10wt. and is Allison C4 rated. And I will let my truck warm up 20 minutes in the winter regardless of the oil used in the transmission.
I just installed a newly rebuilt MT654cr in my truck, it came with dyno test papers and a MATES rebuild tag. It had a large tag on it stating "NO OIL" , use only OE/HDO 10wt. oil. This is what they dyno tested it with so I feel pretty confident about my choice of oil.
if I were using my truck off road, driving very slow, meaning 1st-2nd gear when the torque converter stays unlocked generating more heat, with minimal air flow over the radiator, and the ambient temperature stayed over 95 degrees F (think desert) then I would use 15-40w motor oil in my transmission.