I'm planning on changing the ATF fluid and filter on my 5.3 v8 GMC envoy, I have lots of extra Max Life ATF (3 gals) due to having a BMW with GM/ZF tranny which is compatible with valvoline Max Life. Will Max Life ATF work or it should be pure Dexron VI ?
Max life. Many people get in a huff about multi vehicle ATF. I have a mix of Maxlife and Amsoil in my trans since the summer of 2014, the thing still works great and has not blown up yet. Neither of them were licenced Dex 6. Oh the horror.
Ran a multi vehicle ATF in my old 2011 Ram 1500. Not licenced ATF+4 but suitable for use. Guess what? Ran like a dream.
Probably a losing battle to compare slushboxes in a GM owner forum...lol. Of all the things GM got wrong over the decades, the auto tranny is one of the few things they did well (or, at least 'more' well than pretty much all of their competitors). JMNHO
So back to the original question. The OP's truck is an '03 which specified Dex III at the time. Dex VI is the new and improved version and would be good. Max life, if it specifies Dex III or VI compatible would be fine.
The max life atf has synthetic and dexron vi printed on the jug. I flushed the transmission and have been running with 10 quarts of it for the past year with no issues. I did the same with a lumina that was having shifting issues - after a few weeks with max life the shifting returned to normal.
And the price is competitive too!