If you dig deeper into viscovity, you'll see why Dexron VI is backwards compatible with Dexron III, and fully recommended by GM for their transmission and most hydraulic systems.
Although Dexron III starts off more viscous, it fairly quickly becomes less so after a few 100 hours, stabilizing at a lower viscosity (about where Dexron VI starts). Dexron VI starts off lower viscosity than Dexron III, but pretty much maintains it over thousands of hours. Dexron VI is also superior in every other way to Dexron III; better shear tolerance, flash point, lubricity, metal wear, etc.
My '02 HL has been perfectly happy with Dex VI for three years.
What do '07 HL's (the last to have hydraulic steering) use for PS fluid?