What type fluid does my mod. 917.251561 craftsman lawn tractors hydrostatic rear end take?

Owners manual, see page 19.

CRAFTSMAN 917.251561 OWNER'S MANUAL Pdf Download. (http://www.manualslib.com/manual/480160/Craftsman-917-251561.html)

Hi,
That is a good place to get the manual, but it will not answer the question. It is a Hydrgear unit with replaeable filter. Most of these units now use a 10W30 oil of SH rating or higher.
Peace,
Clarke

go to nearest reputable lawn equipment parts and dealer. Xmark Z trans fluid works great