Any p/s fluid, Wal Mart or otherwise will work fine in these older systems. The much newer systems, like late nineties I think changed to seals that work with ATF.

There is nothing wrong with changing seals and giving it a shot. You need to inspect the shaft though. Often it will have a groove especially if ATF was run for an extended length of time.

Once the system starts leaking with ATF, changing to PS fluid will probably not correct anything.

Good luck,