Welcome to the forum!
When it slows down going up a hill, does it sound what I can only best describe as "brassy" like a trumpet (loud screaming) or a whine like a regular hydro?
When it rolls down here, do you here a buzzing sound like it is freewheeling or does it whine as it holds you back from going for a fast ride!
One of my favorite tests is to push up the tractor against a tree or a building (harder to do with a traditional tractor than a zero turn without tearing something up) and seeing how well the wheels break traction and spin. You'll learn a lot that way about your hydro.
I wouldn't panic yet. Sometimes hydrostat problems are corrected and cured by a fluid change and filter change.
Well it whines a little and significantly slows down, freewheeling or slipping (tranny not Tires) describes the sensation that I get. some of the areas that I mow are "technical" so I'm going pretty slow to begin with, when I push the speed forward to power through the slow down the whine goes away, then the tractor takes off after the hill or hard turn is over which is unnerving .
That should be item one on your list of things to do, unless it was done before you bought it. And you believe the seller when he said it was done.
How does the fluid look in the sight tube on the backside of the tractor? Clear/brown or red? In other words, does it have ATF or Low Viscosity HighGuard in it right now
Clear best describes the color, it shows 3/4 full when cold, then drops out of sight when running for a bit. I bought some Lucas Uni-Hydro Fluid which says it meets the standard for Hy-Gard. Then I read on forums about how not to mix and got scared. Should I top it off just to test, then do a full change and filter?
yes its normal but just dont run the throttle too high you could blow the engine
Where do you recommend running the throttle?