Many say "don't change tranny oil" but then what?

It absolutely is a GM tranny...a French built 5l40e....

If my memory is right...8 qts. It's in the DIY above.

As a side note....

I had a gmc pickup with the predecessor tranny...the venerable 4l60e and got a little over 300k miles with fluid changes every 25k. It's an auto so YMMV but the General knows what's its doing after all....they've been building automatic for an extremely long time....Heck hot roders still love using the turbo 350 . And that tranny brings back fond memories of getting scratch at fwy speeds by hitting the accelerator hard in my dads 68 firebird conv.

Thanks again B.

I swear GMC pickups just last forever... I had a buddy with 250k miles all stock, manual tranny, and that thing just ran over everything... *sigh* wish these cars would be built that robust

I'll start prepping to buy the fluid.. just so you know I have read those DIY's but it almost turns out to be a debate as to what fluid to get. Having said that, I have noticed two things:

1) The fluid is Dexron-VI (8 quarts as you said)
2) The fluid is dark-brown rather than the usual red ATF

I'll do a little more research and see if my dealer carries it, I really would love to stick to OEM if possible. It's funny though, I have read more discussion about what ATF to use than what engine oil, gasoline, and differential fluid to use all combined. lol