Lucas Heavy-Duty Oil Stabilizer, STP Oil Treatment, Motor Medic Valve Medic and K & W Posi-Trac Gear Oil Additive are good brands of additives. As of July 2015, they all average more than four stars on five-star rating systems at Advance Auto Parts' or Autozone's website.
At Advance Auto Parts, reviewers state that the Lucas Heavy-Duty stabilizer reduces engine oil, stops leaks and extends oil change intervals. Comments note that STP reduces oil consumption and engine wear. According to reviews, Motor Medic's additive helps the engine run clean and loosens sticky valves. At Autozone's site, commentators remark that K & W's additive is useful for certain oils in order for vehicles to run quietly.