According to tractor data
TractorData.com Scotts S1642 tractor information your tractor came with a tuff torque K 51.
According to the Tuff Torque web site
https://www.tufftorqservices.com/EnvEEdefault/FlatHTML/TechInfo/ttcoil/oilhelp.html your box uses 5W40 fully synthetic.
Manuals on line have a free downloadable owners manual which should also contain the essential service information . Download it and consider the maintenance schedule as the MINIMUM requires servicing schedule & the moweer will run for 30 years.
Scotts Lawn Tractor Operator's Manual | ManualsOnline.com.
Now all this assumes some one like me as not been in there swapped out the tranny and resold the tractor I bought because the tranny was blown.
Tractor data is a site you might want to book mark ( add to favourites ) really good site for comparing the specs of the older model tractors you find in the used tractor market.
When you buy a new trans they came with about 1/2 dozen identification stickers which are just a bar code, with the model & serial numbers under neath .
They are quite small about 1/2 tall and 1 to 2 inches long.
I am supposed to put one next to the tractors id plate, one under the seat and one at each end so when you take the wheels off you see the sticker and one on the trans under the pulley and one by the brake ( if it has one ).