DO NOT use skydrol (aircraft fluid)! This will ruin the entire system! If the system isn't designed to use Skydrol it will eat and destroy every part in the system that isn't metal! (I work at a major aircraft company and see what skydrol does to stuff... and that is turn stuff to goo rubber, plastic, paint)
The system is designed for the aircraft fluids that Aleigh listed, and you DO want to use them. They contain corrosion inhibitors that will keep the system from rusting out from the inside, as a number of owners have found while using regular hydraulic fluid. There is no particular design flaw that allows water in (as Aleigh suggested), because there is supposed to be a metal cover over the whole rear portion of the pump that shields the vent... if you're missing it then that is your problem.I figured out you can by hydraulic oil by the gallon and 5 gallon pail. I just added some to bring the level back up. I'm not going to buy any special fluids unless it's required. It's actually novel to be able to buy fluids anywhere.
The fluid is more expensive than regular tractor hydraulic fluid, but not a big deal in the context of everything else on this truck. I think I bought it online for $6/qt, and the whole system uses 3qt. I think I got free shipping at $50, so I went ahead and bought 9 quarts, and have enough for a very long time now.