Resurrecting a dead topic, here is video I came across, its a flat tappet cam Vs roller bearing cams and mentions ZDDP ( Zinc ) oil if you want to run a flat tappet cam, also useful for people who have no idea what flat or roller cams are.
It comes up erratically. Sometimes at lower speed, sometimes at higher speed. When the vibration is there, it can be felt not only on the steering but also on the seats. There is no vibration when the car is not moving.