Actually, 3D softwares struggle to rotate an object that is already tilted on another of its axis. That's why it's better to keep the object straight, rotate it on the wanted axis, parent it to an empty, and tilt the empty, not the object itself.
Also, take note that the animation won’t take your 3D View Pivot Center setting into account, it will use the object's origin.
About the rotation value: you should choose 180°, then, in the Graph Editor, shiftELinear Extrapolation so that it understands that it needs to go indefinitely with a constant speed.
