Well, my PS is working better with the synthetic fluid than old dirty Prestone - less noise (but still some) and no stiffness in the morning. However, I noticed yesterday that my PS pump still burped the fluid (I see a puddle on the driveway) when it was -15F the other morning. I attribute this to the fact that I have the extra 24 feet of hydraulic hose in my system that goes to the winch, and it puts up extra resistance to flow, especially noticeable when it is cold and fluid viscosity is increased.
I will try to add some Lubegard to see if that will help. Adding PowerService to the pump completely eliminated the noise, stiffness and burping of fluid but I am not sure how safe the fuel anti gel is for the PS system. Lets see if Lubegard is as good as PowerService

 

if the syn oil or lubguard does not do the trick you might have to look at the hydroboost being at fault, does your HB pass the tests

 

I just replaced my PS fluid the other day as well. Mine was so stiff when it was cold that you couldnt get the brake peddle down or steer.
Right or wrong the way i did it was take the line off the hydroboost closest to the fender and put a hose on it and stuck it in a jug. I had a buddy start the engine while i poured ATF+4 in the pump until good fluid came out, then i hooked the line back up filled the pump steered side to side stepped on the brakes, and took the line off until i had good fluid again. I didnt realize how bad it had been steering even when it was warm, its silent and effortless steering now.
Just as a side note ATF+4 is actually an acceptable PS fluid for this system.