It's been shown conclusively that the most effective gasoline additive, the best at cleaning deposits and eliminating pings/knocks, is not to use an additive at all, but rather to simply switch brands of gasolines every 5000 miles. If you are a brand loyalist (and a lot of people are) and will only use this or that brand, then you probably should use some kind of carbon deposit cleaner additive, but you need to change the brand of additive every time you use one, then.
All gasolines are refined and engineered to certain octane levels and other specifications. Base gasolines, right out of the refinery, are all basically the same. It's a commodity. Oil companies swap base gasoline all the time. Texaco gasoline may have come from a Exxon refinery and Exxon gasoline may have come from a Shell refinery, and vice-versa badda bing. For example, at the Exxon refinery in Houston, you may see trucks from a dozen different companies, Chevron, Shell, BP, etc., all loading at the
same Exxon terminal at the
same time. That's the base gasolines they are loading up with, and not the branded gasolines. What comes next, however, is what makes the brands different, it's what makes Texaco Texaco and Shell Shell. The additives.
Each company has its own additive and adds it to the base gasoline. So while the base gasoline may be the same, the additive is different, and hence the brand of gasoline you use is different because of the additive, not the base gasoline. The additives in each brand are chemically and molecularly different, and thus will clean and lubricate differently.
The additives, whether already in the gasoline or something you add, will leave its own unique type and molecular deposits, different than base gasoline alone will. Your engine will build some kind of deposit based upon what additive you have in the fuel, be it added by your or by the oil company. An additive will not generally clean its own deposits, since the deposits made by the additive are not foreign to the additive. A different additive will. It'll see the deposits as foreign and will clean it right up.
5000 miles is just easy to remember. You can switch brands sooner than that, but you should go at least 5 tank fulls between swapping, or about 2000 miles (otherwise you're just mixing additives in the same tank). Once you've switched brands, the vast majority of deposit cleanup with the new brand will have occurred after about 1000 miles, and the rest will get cleaned out by 2000 miles. After that, the new deposits start to form, and around the 5000 mile mark is a good place to switch again.
No need to make it complicated, as switching between 3-4 brands is enough. Three, really. You want at least three because you could be switching back and forth between two brands that both use the same or very similar additives. The companies that tout their additives as being best will all use different additives. Shell, Exxon, Chevron, BP, Texaco, Conoco-Phillips, QuickTrip and Union 76 all use different additives (courtesy of a Corvette Usenet newsgroup where the members are really anal about this stuff). Pick three, like run Shell for 5000 miles, then switch to Exxon for 5000, then switch to Chevron for 5000, and then start all over again. Or, go to a 4th or 5th brand if you like. The key is to consistently switch additives to clean out the deposits, and keep them clean.
Unfortunately, or fortunately, depending on how you look at it, the refiners and retailers of diesel aren't allowed to add detergent additives, so it's up to the consumer to use the proper fuel treatments.
Incidentally, here's the MSDS on
MARVEL MYSTERY OIL.
BENZENE, 1,2-DICHLORO- (solvent that increases octane and reduces knocking, same as putting a higher octane gasoline in the tank. A carcinogen, currently limited to 1% in gasolines, will be reduced to .62% in 2010 by the EPA)
MINERAL SPIRITS (solvent, particularly good at removing carbon from metal)
NAPTHENIC HYDROCARBONS (a hydrogenated benzene, also known as cycloparaffin, a lubricant oil)
