My "new" '06 hasn't had the tranny fluid changed, as far as I know (it has 20K miles).
What fluid is installed at the factory?
If it's not already, should I be using Transynd? What are the benefits?

Should I switch to the deep pan/filter?

TIA

Dexron 6 from the factory. Some use Transynd, some use the Amsoil equivalent and likely some even use Dexron 6. If you use Dexron 6 then GM couldn't bother you if a warranty issue arises. Then again are they going to test and see which fluid you are using? Don't know!

I would go with either transyn or amsoil torque drive for the denifits, the cost is higher though. If you go with dex 6 and just change the fluid often you'll be fine.

Is this a "the sooner the better" kind of thing?
The truck has 20K miles on it now.
Should I switch to Transynd now?

transynd if you use that accordign to allison's website can greatly extend your drain intervals, will offer great protection