First off, power steering fluid, like transmission fluid, is 'Hydraulic Fluid'. Dexron II was the standard when your car was built. Since then the manufacturers of Dexron have improved their product to meet new car standards - hence 'Dexron VI' that you're seeing in the stores today. Dexron VI is backwards-compatible with earlier specs.
Now, about the reservoir always emptying... If you keep filling it and then finding it empty again - I think you already know the answer to the question about a leak. Right?
Look under the car and see if you have a wet streak around the power steering pump, lines or rack. You might be able to eyeball the cause of the leak and either fix it or get it fixed.
In the meantime, and this is a trick my granddad taught me, pour about a tablespoon of brake fluid into the power steering reservoir (yes, brake fluid). Brake fluid can cause seals to swell enough to sometimes stop (or slow down) leaks. I used this trick in an old Mustang and it worked like a charm - never leaked a drop again.
Go on now - go take a peek under the car...