I had a PS issue in my 07 focus and got to a point I was burning up pumps left and right... I let the fluid drain, ran it that way for 2 years no issues... One day about a year ago I added a tranny cooler to the PS return line and refilled it and worked no issues... SO Going by that, no fluid will work for as long as you want.. just remember to have your vehicle moving when turning, try to never over exert the system by standing still and turning the wheel. that will stress the gears. This is in Vegas where heat is a issue and I had no issues with manual steering
there is one issue... the steering wheel column has 2 plastic washers/guides that will get stressed from the heavy turning and MAY need to be replaced after a few years but thats all..
I just torqued mine down better.