ATF vs. Power Steering fluid

[Tempest]

Is there really a big functional difference between fluids when used in P/S systems? I know ATF has friction modifiers that are not needed in P/S, but the cSt for most P/S fluids is ~7 @ 100C which is in line with most ATF's. ATF is pretty much hydraulic fluid anyway, which is exactly the type of system P/S is.

Is there any real reason these cannot be interchanged with each other regardless of what is spec'd?

 

[Whitewolf]

There are some very large differences in some cases.
Some P/S systems do use ATF but even then it's best to use only what is recommended as ther might be material compatibility issues otherwise.
Some systems use special fluids that may look like ATF from a viscosity aspect, but they are not. The Pentosin products are an example.