[Kube]

The hexagon plug is the lubrication plug.
If the steering is stiff, it can be a couple of things.
Yes, first check the fluid level. If that's okay, check the adjustment. By the appearance of the screw, I'd guess someone has been tightening it down. Perhaps too tight?

Rebound? If you mean the steering doesn't straighten back out easily after a turn, I'd check the toe-in and well, have the front chassis professionally aligned.