I haven't used Amzoil Powershift, but what the mfg will tell you to use is a straight mineral oil. Chevron, Lubriplate and Cat have it. If you use a standard gear lube, after a while it will put a glaze on the steering brake pads and that's then end of making it turn. You can take them out and rough them up with a wire wheel on a small side grinder, but that's just a temporary fix.
Bill