make/model of both the rider and the sick tranny would help.

You beat me to it!

Just went out and took the bottom / pan off the old drive
and saw that the control tilts a plate one way or the other depending on forward or reverse, and the plate pushes down on what looks like round pistons that probly act as valves to direct the fluid flow? No adjustments in sight.

The mower is an Ariens EZR Zero Turn and the drives are
Hydro Gear 015404.

If the new one has the same problems as the old one, I would guess the tolerances just weren't correct when the drive was built and there's no remedy except rebuild or replace? And counting shipping and labor and parts, probly cheaper to replace? I'll try some 20W50 synthetic first,
and if it's still weak I'll add in some light weight synthetic diff oil. Nothing to lose at this point probly.

Ah HA! Methinks I've found the problem.

Was in the process of dropping the drive assembly down to change oil, starting with disconnecting the linkage at the drives.

Just to check, I moved the control levers and found the left one (the problem one) didn't want to move forward as far as
the right lever, so checked into that further. I disconnected the linkage at the hydro unit and moved it by hand.

On the right side, which works properly, the control shaft going into the hydrostatic will rotate further for Forward than for Reverse. On the left side, the shaft rotates further in Reverse than Forward. Apparently something's backwards inside. Then I noticed that my "new" trans axles have a small bead of black silicone squeezing out at the pan mating surface - don't think that was a factory sealing method! My old ones sure didn't have that. Hard telling what went on with that.

End result is that I got a used or rebuilt set of transaxles and it was screwed up somehow so the left side unit operates like the right side unit, but of course that doesn't do me any good for the left side.

And heavier oil isn't going to help, either, as it is set up so forward and reverse travel speeds are switched.

BTW - the vendor was yanxin1105@hotmail.com and the units were shipped out of Sutech in Galesburg IL.

Yes, yes, that's ebay crap for you I guess. Just couldn't afford dealer prices which would have been more than a replacement used mower.

At least I've figured it out, but now can't mow any faster than what the mower normally would travel in reverse! Guess I'll have to finish replacing the hydro power unit in my 60" and go back to using this 42" as my trim mower again.

Probably can take that one apart (carefully) and flip the control thingy inside, seems I was reading somewhere here or on another site someone had put one in backwards in a regular hydrogear t-40somthing and it had slow fwd and fast reverse.

Are the part numbers different for the left and right side hydros? Is it possible that you have two right side units? I don't know anything about ZTRs, but is this a possibility?