I used Dextron 3 (AT Fluid) still avail at most auto parts stores.

 

Dextron III Mercon. You could run Mercon V but I would suggest draining the entire system if you planned to go that route. Mercon V is the synthetic version of Mercon III and it is backwards compatible with anything that uses Mercon III.

(You'll probably have an easier time finding Mercon V as a lot of manufacturers stopped using Mercon III in transmissions about 10-15 years ago.)

 

I have used Mobil 1 Transmission fluid it is a synthetic....

 

It's going to be hard to find Dextron anything, it's Dexron.
Dexron is GM's trademarked name for ATF, while Mercon is Ford's.
In the 70's/80's/90's it made a difference, but now they are pretty much the same.
Dex III and Merc V meet the same standards, and either should be easy to find.

 

I've been using Dexron VI for three years. Works great.

 

Find yourself a transmission fluid that is Dex III compatible. Mobil 1 as someone has mentioned earlier is, it's what I use. I'm not saying go get Mobil 1, its just an example. You could use Dex VI as AVConsult mentioned, the only difference between the two is that Dex VI has a slightly lighter viscosity (thinner fluid, but not by much), but that shouldn't be a problem.

 

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?

 

My 07 HL manual says use type II or III Dexron AT fluid for the PS.

 

I figured the '07 would also use Dex III--Dex VI was introduced in 2006. Really, Dex VI is backwards compatible and a greatly superior fluid.

The only time I think you'd want to stick with Dex III is if you had a seeping/leaking rack or worn piston seal, or worn pump, where Dex III, at least when new, might work better....at least until it got a few hundred hours use.