Hastey
Oklahoma Chapter Leaders
All Allison transmissions in GM vehicles come from the factory with synthetic transmission fluid.
to further add to what Terry just said is that if you are now running anything but the DEXRON VI synthetic. I advise change it immediately. Also, carry a spare quart or two with you in case you have to add. You just don't walk into a quickie mart and pick that up.
I've had my Allison out of my truck. It's no fun at all. I had to hire a guy to help me. Even GM puts two mechanics on it at a 12 hour shop rate. 14 hours of labor to put a $9.00 front seal in. Oklahoma Panhandle sand will eat them out of there quick.
The cheapest I found a shop to do it was $1200 plus parts.