Mike, PSFred, Jim:
Thanks for the info and advice. It seems the ATF is indeed too thick when the temperature drops (but is fine once the car warms up), so I want to replace some of it with PS fluid (which is noticeably thinner) to thin it out.
Is it OK to mix ATF with (red) PS fluid? I plan to drain the ATF from the reservior and replace it with PS, but some ATF will obviously still be in the system. My assumption is that this will produce a mixture thinner than pure ATF, but thicker than pure PS.