Bestline Premium Synthetic Oil Treatment, REV-X Engine Oil Treatment Kit, Liqui Moly 2009 Anti-Friction Oil Treatment and Sea Foam SF-16 Motor Treatment are all well reviewed. Their ratings are also based upon a large amount of reviews.
The Sea Foam SF-16 has the added benefit of being able to be added to a fuel tank during an oil change. A lot of reviews, for all products, noted an increase in gas mileage. The products have different purposes. For example, REV-X is intended to be used with cars that have engine startup problems. Liquid Moly is intended to leave an anti-friction coating on the surfaces of the engine, making it run smoother and reducing wear.