STP oil treatment is the best product out there, (providing you use it correctly) It is not intended to be used for oil burning engines as a quick fix or any other quick fix ideas. You put a bottle of it in your engine every time you change your oil.
STP is still average $2.50 - $3.00 a bottle, depending on where you get it and it does the same thing that all these higher priced additives claim to do. I use it, and have been for over 20 years. It contains detergents which enhances the oil's viscosity and keeps the engine clean and sludge free, and when used regularly over a period of time, it will actually "bake" into the metal inside the engine leaving a protective film to reduce wear. It is ok to use it in a new car after the break in period has passed, that will be a sure guarantee that your engine will have a long life providing you change oil regularly. I have worked on engines that has used it and personally seen the positive effects it has. I have not seen anything that outdoes it, and besides, take a moment and think about how long that product has been on the market......................... a long time.