Which of the following should not be present on the brake rotors?

The presence of oil or grease contamination on brake rotors is a significant concern because it can severely impair the braking system's performance. Brake rotors are designed to function under extreme conditions, generating considerable friction to create the stopping power necessary for safe vehicle operation. When oil or grease contaminates the rotor surface, it compromises this friction, leading to reduced braking efficiency and potentially resulting in longer stopping distances or brake failure.

In contrast, minor dust buildup is common and typically does not affect the performance of the brakes severely. Grooves or scoring can indicate wear over time but may still be within an acceptable range depending on the vehicle and usage, as long as the rotors are still within specification. Normal wear marks are expected with use and generally do not signify an immediate concern unless they reach a certain level that denotes the need for replacement. Therefore, keeping oil or grease away from the brake rotors is crucial for ensuring that the braking system operates safely and effectively.