I have a new 400 Z master Toro ZTR. I have made it to the 8 hour Hydraulic oil filter change. A few questions. 1) No where in the owners manual does it say anything about changing the Hydraulic oil itself. Only the filter. Do I not also have to change the oil? 2) If I take off the oil filter won't all the oil run out of the system? So I will be changing all the oil right?