The advice is plain and simple: never use them! There are many oil additives and so called engine treatments (often sarcastically referred to as snake oils) available on the market but they all share one thing in common: none of them was ever proven to be advantageous, actual tests found quite the contrary more than once. Manufacturers routinely claim that independent laboratories tested their products scientifically but they don't rush to disclose the names of those laboratories or the circumstances under which the tests were conducted.
The most popular oil additive is PTFE: this is the same old Teflon you know from your kitchen. As Teflon is a registered trademark of DuPont Chemical Corporation, oil additive manufacturers have to use a substitute name, the acronym for its real chemical description, polytetrafluoraethylene. DuPont issued an official statement ten years ago stating that "Teflon is not useful as an ingredient in oil additives or oils used for internal combustion engines".
The manufacturers claim that PTFE, present as a suspended solid in the oil additive, coats the moving parts of the engine. However, the reason for having an oil filter in the lubrication system is actually to filter out and retain the suspended solids present in the first place. PTFE particles can therefore clog the filter, reduce the oil pressure in the engine and lead to oil starvation and major engine damage. Some manufacturers claim that their PTFE particles are much smaller than the particle size the oil filter is designed to filter out, but they forget to add that PTFE expands radically when exposed to heat. In consequence, even if the particles were small enough originally, they may not be when the engine reaches its operating temperature. Be sure to avoid any product that says "Shake well before use" on the label: if the solids in the product will settle to the bottom of the container while sitting on the shelf, just imagine what will happen inside your engine...
Another material used in those engine treatments is zinc dialkyldithiophosphate (ZDDP). Unlike PTFE, this additive can be found in virtually every engine oil sold today. It is used as an extreme pressure anti-wear agent which does not come into play unless very extreme conditions like revving over the redline zone are present. Research shows that more zinc does not give more protection but can damage the catalytic converter. The amount of zinc to be found in major brand engine oils is a good compromise between engine and converter protection.
The engine oils on the market contain a wide selection of additives already in the mixture. Many of those additives are carefully selected to achieve an effect in groups of two or more that none of them could attain individually. Adding more additives to this formula, even if nothing more than something that was already included in the package will eventually upset the balance and negate the original effect of the mixture.
When quoting favorable opinions about their products, manufacturers often use testimonials of professional racers or teams. Beware of those testimonials: high performance racing presents substantially different requirements on engines. Some of these oil additives are in fact capable of producing less engine friction, improving gas mileage and engine power. Alas, all these come at a price: reduced engine life. While this is not an important issue in high performance racing where the engines are designed to live a very short life (a few races or even only a single one), you would probably not accept those benefits if you had to pay for them with a complete engine rebuild.
And don't believe in infomercials. It's not too complicated to find people who stare with their mouth wide open and praise any product if you happen to pay them generously. Even seemingly convincing tests like draining the oil from the engine and spraying the crankcase with water are bogus. Water is a good cooling agent and lubricant for the engine so it's no wonder and certainly not attributable to their 'revolutionary breakthrough product' that the engine runs smoothly for some time without trouble. Naturally, you wouldn't want to see what remains of that engine after the shooting... And as independent tests show, a usual engine in normal condition can easily run up to 50–100 kilometers with the oil removed (but, of course, will be completely destroyed in the end), without the need for any wonder oil treatment.
The major oil companies are rich, powerful and aggressive corporations, with significantly more muscle power than independent additive companies. With all their capabilities and resources in research and product development, you can rest assured that if any additional material was really efficient without being harmful, it would have already found its way into the products of those major oil manufacturers a long time ago. In contrast to the unknown laboratories the additive manufacturers claim to endorse their products, well-known independent research laboratories, state universities, major engine manufacturers and the NASA did collect a considerable amount of evidence against the effectiveness of those additives.
Recently, Quaker State, Inc.—the company behind the most widespread engine treatment, Slick 50—had to agree to a settlement with the US Federal Trade Comission which bans them from making certain advertising claims for which they have no factual or scientific background. Manufacturers of other products (eg. Motor Up, Dura Lube, STP and Prolong) also had to pay fines or face legal actions. For more information visit the US Federal Trade Commission and use their search engine to look for "oil additives".