Simplicity Tractor fluid

I have a reservoir under the seat of my Simplicity 5212.5 Mower circa 1984 42" deck. I believe it is for the transmission or hydrostatic drive. The manual says to use a special fluid anyone know FOR SURE what it is? a 30W or a 90W or ATF? or straight hydraulic fluid like is in a hydraulic jack? I'd like to change it and have a correct reserve supply. Also is there a manual online to D/L for the 12.5 HP Briggs horizontal opposed engine someone can point me to. tried a few places and they want a Credit Card which I don't want to do. Thank you in advance.
BigSmokeDan

From looking at the manual on page 13 it calls for Power Fluid 821, when I searched it I found a cross reference that I am attaching the link to as well as the manual link. I also noticed that the pump and differenental were separate and need to be check that way but call for the same oil.
https://www.manualslib.com/manual/153349/Simplicity-5212-5h.html?page=17#manual
http://www.docs.citgo.com/msds_pi/988313.pdf