The 12 gallon hydraulic fluid includes the HST (transmission), rear differential & gear box, and hydrauilcs for steering - loader - 3 pt.
Actual capacity listed is 8.1 gl for a HST tractor and 11.4 gl for the SST/gear tractor. Never hurts have extra on hand in case you spring a leak sometime.
All fluids and filters should be changed at 50 hrs. To help flush out and left over machining or wear in debris that has not gone through the filters.
After that, chage the hydraulic & hydrostatic (if HST) at every other engine oil change, and replace hydraulic fluid at every 4th engine oil change.
