Your owner's manual should specify Dexron VI for the 4L85E transmission in your truck. Dex VI is synthetic, so there is no "regular fluid". Dex/Merc is a fitzall fluid for older GM's that use Dexron III and Fords that use Mercon.
GM went to Dexron VI in 2003-ish and Dexron III is now discontinued. Dex VI is now the only GM-approved fluid and is designed to be backwards compatible with all GM transmissions.
I did a flush/fill with filter change early this year and just did all of this research. I used Dex VI and have had no issues.
