I have a 2004 Sable with the Duratec engine. I have 45,000 miles on the car. Recently I took the car to the shop because I wanted to have the power steering fluid changed. They told me there was no need because the fluid looked good. At 45,000 miles should I still change it anyway or just leave in it....and if I do leave the power steering fluid in how much longer can I leave it in?