Lumpy Steering - What fluid is best?

Hi, I'm having ongoing issues with the steering. It seems fine when warm, but lumpy / stiff when the weather is cold. I changed the fluid to ECP ATF DX III, things improved for a while but then went back to as before. I changed the oil again and the same thing happened so I'm wondering if I need something different, maybe fully synth oil?

Any thoughts welcome.

Thanks