Power steering fluid

At that age I would not suggest changing it at all. If you do, fresh fluid will clean out some crud deposits, which will create some leaks, and free gunk up to go through the system and cause problems. Basically, leave it alone unless there is a problem. As far as which fluid to use, the people at the local parts store should be able to look up which oil to use. Hope this helps.

Needing to be topped off means that there is a leak somewhere. You should probably check for a leak.

You may not see a leak depending on where it is. Many racks will start weeping at the seals inside and then leak out into the boot area on the tie rod end. If the boot is hanging low or looks like it has bulged or sagged, use your hand to see if it feels like it is full of liquid. Also, be careful looking for power steering leaks, the pressure in the system can inject the fluid into the skin if there is a small pinhole. A quick way to check is to wipe the hoses down, then dust them with baby powder. Then start the engine and look for the area that discolors.