Sliding friction between the surface of the cam and the cam follower which rides upon it can be considerable. In order to reduce wear at this point, the cam and follower are both surface hardened, and modern motor oils contain additives to reduce sliding friction. The lobes of the camshaft are usually slightly tapered and the faces of the valve lifters slightly domed, causing the lifters to rotate to distribute wear on the parts. The surfaces of the cam and follower are designed to "wear in" together, and therefore each follower should stay with its original cam lobe and never be moved to a different lobe. Some engines (particularly those with steep camshaft lobes) use roller tappets to reduce the sliding friction on the camshaft.