I personally don't like to use ATF in a PS circuit. I only use PS fluid. It doesn't really matter what brand because I drain and fill it at a regular interval. According to Howe, GM fluid is top notch. I torture tested several differnt brands in my '79 Chevy pickup with 44" tires on a Detroit locked Dana 60 running ram assist steering. Believe it or not, best results came from using Pyroil PS fluid from Autozone.