The plastic tank is much like a radiator expansion tank, yes. 20W50 synthetic. Cleanliness is the most important thing here. Hydrogear may have a service sheet they will send you if you ask nice. I don't believe that there is a drain, so in its absence you will be cleaning and inverting the transaxle.

It is out and inverted. What an ugly little job it will be to get it back in place. I made notes, labeled things and took pictures.
I think I'd be fine putting it back together if I could do it right away. I am hoping it drains quickly.

Hydro-gear was not much help but I did discover a hint for the amount of fluid. I will be comparing that to what comes out. My assumption is that it may not be possible to get it all out but that any large amount that is replaced will provide some improvement.

Thanks for the guidance.

2.4 quarts of 20 W 50. Synthetic is best.
I have a PDF service manual if you would care to e-mail me.
Check my forum profile.

Mownie:

Thanks for the kind offer. My e-mail request is en route.

scotfree

Final post for this string.

After reinstalling and several runs at purging the system of air, I have to say the operation was a success. Top speed is slightly increased and the pedal(s) are more responsive than I remember. It is not quite as good as new but it does go uphill better than it has in a long time. Well worth the effort overall. Thanks to those that offered some expertise and the service manual.

Notes:

The fluid coming out was rather black. It took almost the full amount (about 4 oz less) to refill than stated. I think that some was likely hung up in the workings and did not drain. I have spotted a bit of fluid in the overflow bottle after a recent 2 hour mowing run, so I think it is indeed full.

That sucker is way heavier going back in than coming out.