A good (if kinda wordy) read: Oil Additive Study

Probably preaching to the choir, since it's been said here before. I've quoted main points of the article below.
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1) Avco Lycoming, a major manufacturer of aircraft engines, states, "We have tried every additive we could find on the market, and they are all worthless."

2) Briggs and Stratton, renowned builders of some of the most durable engines in the world, says in their report on engine oil additives, "They do not appear to offer any benefits."

3) North Dakota State University conducted tests on oil additives and said in their report, "The theory sounds good- the only problem is that the products simply don't work."

4) Ed Hackett, a chemist at the University of Nevada Desert Research Center, says, "Oil additives should not be used. The oil companies have gone to great lengths to develop an additive package that meets the vehicle's requirements. If you add anything to this oil you may upset the balance and prevent the oil from performing to specification.

In addition, all of the major vehicle and engine manufacturers spend millions of dollars each year trying to increase the longevity of their products, and millions more paying off warranty claims when their products fail. It only stands to reason that if they thought any of these additives would increase the life or improve the performance of their engines, they would be actively using and selling them - or at least endorsing their use.

Instead, many of them advise against the use of these additives and, in some cases, threaten to void their warranty coverage if such things are found to be used in their products.
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eek Says it all to me. [Wave]