Thinking of flushing my Alli at 75000mi with Mobil 1 Synthetic ATF. I currently have the Mobil 1 in my transfer case. Milage on the truck is about 67000 and towing a 10000lbs 5th wheel.

That's what I run and it is on the approved recommended fluid list for the Allison transmission,Dextron III or equivalent.

There is a DIY on a flush in that forum and plenty of reading here in the Allison forum on change ,capacity and additional flush procedure or information.

Just a search away