Image Stabilization is all well and good, but frequently when shooting motorsport you actually want the camera to move when taking a photo. Thankfully, both the in-body system from Olympus and the lens-equipped system from Panasonic both allow you to switch IS off. An alternative is to use one of the modes specifically designed for panning, these only try to remove any vertical movement, leaving your horizontal panning movement alone.
Panasonic: If using a lens equipped with MEGA O.I.S, ensure you switch the Stabilizer mode to Panning (a hand with an up and down arrow)
Olympus: Select the IS.2 setting in the camera menus
Whether you switch it off or use the Panning mode is up to you, I prefer to switch it off as the slight movements noticeable in the viewfinder introduced by IS can be off-putting when trying to pan smoothly.