That Valvoline Dex/Merc in the blue bottle that he posted is the most affordable and readily-available Dexron III equivalent in the U.S. It has the proper (high) viscosity of original Dex III. The synthetic Valvoline Dex/Merc in the red bottle has too low a viscosity so I don't recommend it. Synthetic fluids that I know to be good Dex III equivalents are Mobil 1 ATF and Redline Oil D4 ATF.
General Motors claims Dexron VI is backward compatible for applications for which Dexron IIE/III were originally specified, but its viscosity is significantly lower than that of original Dex III and I know from personal experience that it does not work properly in the ZF 4HP24 transmissions in X300s, so I don't recommend it for the power steering system either. You may not notice a difference using a thinner fluid in your P/S system, but the system pressures may be incorrect and therefore unhealthy over the long term.
