I just checked my book. They recommend 20W-40 motor oil. A little thinking will tell you they pick up that oil out of thetransmission and the pump is clear in the front of the tractor. I always tell my hydraulic classes that hydraulic pumps are not good vacuum pumps. The general rule is 5 inches of vacuum on the inlet side and you can expect pump caviation, not good. That milk color tells me you have moisture and I wonder if you have formed little particles of ice on that metal strainer. If you have hydraulics after being in the warm garage I would say moisture is causing the problem. did you here any noises when you try to work the three point?  Questions?