The Bottom Line Don't use any oil additives whatsoever, NONE of them are effective and many are damaging. Simply use a good quality oil, its got all the additives you need.
Everybody and their brother has some sort of miracle oil additive that promises to add horsepower, recondition your engine, increase fuel economy, and protect your engine from anything and everything. Listen to that claim, it even sounds like they're selling snake oil! Unfortunately they are. Prolong, Slick50, Duralube, zMax, etc have all been sued by the FTC for deceptive product marketing, as they have all been tested and none actually showed the benefits they tout. In some cases (Prolong in particular), the oil additives were actually more harmful for the engine then using oil alone. What about your precious zMax? Nothing more then tinted mineral oil. I'm serious. How can this be? There are absolutely no standards or regulations governing the safety or effectiveness of oil additives. Oil additives fall into four categories: pure snake oil, PTFE, Zinc dialkyldithiophosphate, and detergent-based. The snake oil category are simply those products that are a complete sham, like zMax, Slick50 and Prolong. Don't believe me?
The second category are the additives that tout "PTFE" or "Teflon" to us lamen. The companies that sell PTFE products brag that it coats the moving parts of the engine and reduces friction and wear. Lets think about this a second. PTFE is a fairly large solid particle (flake). Isn't the oil filter supposed to filter our particulate matter down to 10 microns? So won't the oil filter filter out all the precious PTFE that you just put in? The answer to both those questions is yes. Not only is this an ineffective additive, its also dangerous. Lab testing found that PTFE was less likely to coat the moving parts and more likely to begin clogging pipes and other small passageways (not to mention your filter). Not only that, but PTFE was shown to double iron contamination in the oil (which increases engine wear). For the final nail in the PTFE (Teflon) coffin, NASA studied it for potential use in its rockets and found that it was both ineffictive in lubrication and also clogged small passages (like cholesterol in arteries). The following are SOME (not all) of the PTFE containing products currently on the market: Slick 50, Liquid Ring, Lubrilon, Microlon, Matrix, Petrolon (same company as Slick 50), QMl, and T-Plus (K-Mart).
The next oil additive category is Zinc dialkyldithiophosphate, we'll just call it zinc. Zinc is contained as part of the standard additive package in virtually every major brand of engine oil sold today. Organic zinc compounds are used as extreme pressure, anti-wear additives, and are therefore found in larger amounts in oils specifically blended for high-revving, turbocharged or racing applications. The zinc in your oil comes into play only when there is actual metal-to-metal contact within your engine, which should never occur under normal operating conditions. However, if you race, or occasionally play tag with the redline on the tach, the zinc is your last line of defense. Under extreme conditions, the zinc compounds react with the metal to prevent scuffing, particularly between cylinder bores and piston rings. Adding extra zinc via oil additives is not only unnecessary, it is harmful to the catalytic converter. The following are SOME (not all) of the zinc dialkyldithiophosphate containing products currently on the market: Mechanics Brand Engine Tune Up, K Mart Super Oil Treatment, and STP Engine Treatment With XEP2.
The final oil additive category is detergent based. Many of the older, better-known oil treatments on the market do not make claims nearly so lavish as the new upstarts. Old standbys like Bardahl, Rislone and Marvel Mystery Oil, instead offer things like "quieter lifters," "reduced oil burning" and a "cleaner engine." Most of these products are made up of solvents and detergents designed to dissolve sludge and carbon deposits inside your engine so they can be flushed or burned out. Wynn's Friction Proofing Oil, for example, is 83 percent kerosene. Other brands use naphthalene, xylene, acetone and isopropanol. Usually, these ingredients will be found in a base of standard mineral oil. In general, these products are designed to do just the opposite of what the PTFE and zinc phosphate additives claim to do. Instead of leaving behind a "coating" or a "plating" on your engine surfaces, they are designed to strip away such things. All of these products will strip sludge and deposits out and clean up your engine, particularly if it is an older, abused one. The problem is, unless you have some way of determining just how much is needed to remove your deposits without going any further, such solvents also can strip away the boundary lubrication layer provided by your oil. Overuse of solvents is an easy trap to fall into, and one which can promote harmful metal-to-metal contact within your engine. As a general rule of thumb these products had their place and were at least moderately useful on older automobile and motorcycle engines of the Fifties and Sixties, but are basically unneeded on the more efficient engine designs of the past two decades.
Recommendation: All of these products are just psychological placebos. Don't use any oil additives whatsoever, NONE of them have been shown to be effective (except detergent-based additives, which are not necessary except for single use in old and neglected engines) and many are even detrimental. Simply use a good quality oil, it will have all the additives your engine needs, mixed right in. Oil additives only serve to lessen the effectiveness of your oil's additives and lubrication properties.
You can find the above information and additional vehicle maintenance tips/recommendations in the maintenance section of my website: http://www.projectz28.com/ I will be updating it and epinions as time goes on, so check back for updates.