OIL ADDITIVES, WHAT A CROCK?!  So, I have done some research. . .

Blue Corral, the manufacturers of the Slick 50 engine oil additive, have been banned by a Federal Commission from making claims about reduced engine wear, increased fuel economy and lower running temperatures in it's advertising in America. The Federal Commission found the company's claims of increased performance and reduced wear were unsubstantiated, and Blue Corral has agreed to pay upwards of $20M in damages to affected customers.
Source: Max Power magazine, March 1998

The manufacturers of the ProLong engine additive have been dealt a smack in the face by a Consumer Reports Magazine report into their product. CR attempted to reproduce the "no oil" test where all the oil was drained out of an engine which had been treated with ProLong, and then the engine was run. CR managed a maximum of 13 seconds running out of each of two engine before they seized up, welding the pistons to the barrels. The case is being brought to a Federal Commision for prosecution for false advertising claims.
Source: Consumer Reports, October 1998

The manufacturers of the DuraLube engine additive have also been dealt a smack in the face by a Car & Driver Magazine report into their product. C&D tried the same tests as Consumer Reports did on ProLong, and had similar results, but in a much quicker time. The C&D engines lasted a staggering 11 seconds without oil. You do the math. The Federal Commission has been handed the details and are 'processing' it.

See: http://www.ftc.gov/opa/2000/03/motor.htm

AVBLEND is an FAA approved pure micro-lubricant with a scientifically modified molecular structure. This unique reformulation process allows AVBLEND to penetrate and soak into metal surfaces.

AVBLEND keeps friction surfaces cool during break-in period. One of the most important functions performed by AVBLEND is its ability to keep friction surfaces relatively cool while the metal is new -- during the break-in period.

Too much heat during this critical period while the metal is forming itself into its job can easily cause brittleness, loss of flexibility and a tendency to break under sudden and severe strains. AVBLEND allows metal to temper itself under working conditions and assume the right degree of suppleness and hardness for the job it is intended to do during its entire performance life. This is known as work tempering (energizing) and can only be accomplished under conditions where excess frictional heat is eliminated.

AVBLEND allows proper break-in to occur while reducing conditions for cylinder wall glazing during the initial hours of a new engine's life.

I have found nothing bad about this stuff to date.  Can also be used in Aircraft engines.

Click here for zMAZ info!The latest in, well, not actually an Oil Additive!

Here is another one for ya!

Engine treatments containing high levels of chlorinated substances such as chlorinated paraffins, are being advertised as "new, space-age technology." Actually, chlorinated "extreme pressure" lubricants have been used in industrial applications for decades. However, these substances were never intended for use in automotive engines due to their inherent corrosive nature.

Engine treatments that contain solid particles suspended in a carrier oil are classified as colloids. They are also known as solid-film lubricants. This category includes Teflon® or PTFE (polytetrafluoroethylene-a plastic-like resin), molybdenum disulfide (commonly known as Moly) and graphite.

These products employ an entirely mechanical procedure known as burnishing to deposit solid particles onto metal surfaces. And since there are no chemical processes involved, this action is mostly uncontrolled and therefore unpredictable. Consequently, engineered tolerances may be affected. Also, solid particles can restrict the flow of oil through critical passages, galleries and the filter leading to costly repairs.

Remember, PTFE in oil additives is a suspended solid. Now think about why you have an oil filter on your engine. To remove suspended solids, right? Right. Therefore it would seem to follow that if your oil filter is doing its job, it will collect as much of the PTFE as possible, as quickly as possible. This can result in a clogged oil filter and decreased oil pressure throughout your engine.

In spite of the fact that PTFE-based engine treatments have been available for many years, even their manufacturers have yet to agree on basic issues such as how much PTFE should be added to an engine and how long a single treatment will last.

Therefore, in the absence of generally accepted standards, conclusive test results and even industry-wide support for these products, there is simply no reason to buy any conventional oil additive or engine treatment now that a truly superior alternative exists. 

Magnetic Oils:
Pardon? Sounds like something out of Star Trek doesn't it? Well there's an increasing trend in the industry now to try something a little different. It seems that a couple of the big players are now experimenting with charging oil molecules which will attract themselves to engine parts with no other additives. The idea is, to simplify it, magnetic oil. The oil sticks to the engine parts when the engine is turned off, and is theoretically still there, ready to protect when you next turn the engine on. The first big player on the scene with this was Burmah Castrol with their Castrol Magnatec oil. Theoretically, this type of product is a far safer bet than an additive, and so far I've heard nothing but glowing reports about it. But just stop and think for a moment - why have these companies now decided to go this route? Are they admitting that the older additives with suspended solids in them were perhaps not such a miracle after all?

Just take a look at your supermarket stores.  The oil additives have been falling in price steadily, and are now offering a complete care package (Oil, Trans, Fuel & Collant additives in one nice package) for the same price as the single additive was when it first came out.

Do a search on yahoo "engine treatments", and/or "oil additives" and you'll see more. . .