1610 hydrostatic tranny

[MFP8055]

I'm getting my 1610 ready for another summer of upstate NY grass cutting. Over the last few years I've noticed an increasing amount of noise from the tranny/rear end. No performance issues, but a lot of noise. I took off most of the sheet metal to clean up the machine and found the fins on the drive unit clogged badly. The fan was good, but I'm sure it was not doing much to cool the drive. I plan to power wash everything, and replace the drive fluid with 5W-20 oil. It probably has never been replaced.
Question #1: Is the plug at the bottom of the differential the only drain needed to drop the fluid out of the system?
Question #2: I found a small diameter rubber tube that had come off the drive unit. It extended up to the dasboard, but was damaged in at least one area. Is this a vent for the unit?
Question #3: My choice of fluid was based on threads posted in the past. Without opening the whole discussion again, is there a major problem with my choice of 5W-20?
Question #4: Finally, is there anything else I should do regarding the grinding short of a complete rebuild? It seems to work fine, just noisy.

 

[art]

Are there any signs that the reare end might be a replacement? If it has not it is in the right vintage to need some extra attention internally.

 

[MFP8055]

The phrase "extra attention internally" sounds ominous. What exactly are we talking about here? The tranny does make noise, but pulls very well with no slippage. I agree that, given its age, it may not be in the best condition. I've also found the cooling fins packed with debris, so it has been most likely running hot. I thought I'd start with an oil change and see where that gets me. My understanding is that these hydrostatic drives are very complicated, as well as difficult and expensive to overhaul.