I will not disagree with what the manual says but I remember using Hy-guard. I wonder if it is in a DTAC service bulletin?? I just have the regular manual. Does anyone have the Pink or red shop copy??? They many times have different information in them.
It could very well be the o-rings. I have had issues with them before on other machines. They usually would leak setting.
Also the transmission pump will work if you put the shifter in neutral and then shove the hydro full ahead. I often do this in the mornings while I am letting the combine warm up if it is real cold. This would let you get under it and check for leaks/pressure/problems.