To find the best oil additive for your vehicle, check each additive's ingredients, and choose one with ingredients that provide the benefits you want for your vehicle. Research the additive before you use it, as some don't provide any benefits or even harm the engine.
Common oil additive ingredients include viscosity index improvers, detergents, dispersants, antiwear agents and friction modifiers. Viscosity index improvers prevent oil from thinning as the temperature increases. Detergents prevent deposits, rust and corrosion, while dispersants prevent sludge, varnish and acids. Antiwear agents protect metal surfaces. Friction modifiers lower the friction in the engine, which can improve the vehicle's fuel efficiency.
Other ingredients include pour-point depressants, antioxidants, foam inhibitors and rust inhibitors. Pour-point depressants prevent wax particles in oil from congealing. Antioxidants keep oil from oxidizing, which would make it thicker. Foam inhibitors collapse foam bubbles in oil. Rust inhibitors keep metal parts from rusting.
Engine oil already has additives, so an aftermarket additive is often unnecessary. Some ingredients, or too much of an ingredient, have negative effects on performance. For example, too much dispersant could have a negative effect on catalyst performance and fuel efficiency.
Another issue with oil additives is false claims made about the additive. The Federal Trade Commission charged several oil additive manufacturers with false advertising for unsubstantiated claims.