In terms of Dex III vs the new stuff, the main difference I've noticed is viscosity at lower temps. Both fluids are pretty much identical in terms of viscosity at operating temperatures, but the newer Dex VI at cold temperatures is less thick and closer to operating viscosity than the old stuff. Probably some friction modifiers in there as well. I'd still stick with the older stuff on 97-03 transmissions, but I'll admit it's only because that's what I've heard people doing on here. I've got an 05 so it's all Dex VI for me. Any brand, transmission fluids between brands are almost identical compared to different brand motor oils.
And 02NavyBlue, to me it almost sounds like you have a worn line boost valve causing that pressure drop. If you can access the side cover and remove the channel plate and valve body, Sonnax makes a drop in replacement for the line boost valve and sleeve. The stock ones use a soft aluminum valve that wears away over time and becomes sloppy in the bore, leading to pressure drops. The Sonnax one apparently uses a hardened aluminum valve assembly, with more fluid channels cut to lubricate it better and make it last longer. I'm actually throwing one in myself once my damn valve body gaskets get here. OE Parts HQ, heard good things, but I ordered it last friday and I have yet to even receive a tracking number. If customer support doesn't respond soon, I'll be stopping by a dealership ASAP and telling OE Parts to eff themsel
