Only oil that offers an additive. Vehicles that are taken care of and driven slowly keeps the need to switch to synthetic. Slick 50 seems to have my car run 45% to 55% cooler. Anybody that has the opinion that Quaker State causes sludge I find hard to believe but I've never used Quaker State motor oil without the Slick 50 additive. I'm one of the slowest drivers on the road coming from a stop to the speed limit. Running the vehicle at a lower rpm helps keep the engine cooler,gas mileage,helps oil protect the engine in return less wear and tear on the motor. Still while accelerating this way would cause more sludge build up. My vehicle has never lost power and never caused my fuel mileage to drop. I'd bought a 2000 Buick Century 2 years ago that had 81,000 miles has that ran 20mpg intown 30mpg highway. Using Q.S. Slick 50 my gas mileage stayed at 30.86mpg to 31.71mpg using the interstate 50% of the time. I've never burned oil or had any oil leaks caused by using this product. The oil is allways in good condition during an oil change (2-3000miles). Quaker State does not cause sludge or engaine damage, it prevents it.