Powersteering oil???

Hi all of you...... What kind of oil would you recommend to use in the power steering on a E150

1988 302 EFI? I Use Dexron III automatic transmission oil syntetic, but I think it is kind of thin to use.... Opinion please. Had one Steering gear assembly from the car whas new in the car which I have changed to a new one now. The old one turned to the right by itself after I had changed the O-ring and stuff in the gear assembly. And after I started to use Dexron III. I don't know what kind of oil which was used from the factory. I think is the factory oil which have been in the system until I took it out to change the O-rings and stuff. The new gear assembly is OK, but it is much harder to steer than with the old one. They both look alike, but I think maybe the old assembly is another type because it was ordered when the car was new to a handicapped guy. Thanks for any opinions about this..... Regards.... Tor Elling.