Should have a Tuff Torq K61 in it.
SO off with the tranny, clean the cases like your mothering in law is going to check them.
There is a bung in the top, pry it out crefully tip the unit upside down to drain out all of the oil , will take a long time then refill with 20W 50 fully synthetic.
JD will tell you 5W 30, but this is for a nice new tranny and yours is getting on a bit.
Tranny oil should be replaced every so often 5 to 10 years depending upon useage but no one dose till it is too late.
Tuff Torq have some manuals & instructions on line.
Mulligans seem to have the best prices if you need to replace it.
About 1/3 of the time new clean oil will bring an old unit back to life but it is only temporay once the swash plate & valve chest start to leak they only ever get worse.
That mower is pushing 20 years which is about double the design life of most .
If you get another 5 years out of it you have done well.
Howling hydros are a sign hat they are on the way out but none of my customers will allow me to touch one till it is too late and then when the odd one does they get cranky when it does not sound much quieter than before I changed the oil.
