There's a lot of believers and non-believers when it comes to oil and fuel additives for automotive applications. Asking a group of gearheads "what additive is best?" is a mistake most people will only make once. It's akin to asking them "what oil is best?"

There is no best, everything has its place, its proper application, and some are way better than others, of course, depending what you want to accomplish. Much like an engine oil debate, but with 100x more variables, the topic of oil and fuel system additives is one that can become mind numbing. There's hundreds of different additives, each claiming to be the best, with no real scientific comparison between them.

Many researchers and even YouTubers have made attempts to make somewhat scientific comparisons, however in order to truly replicate the conditions under which these additives are subject to in the real world is a near-impossible feat without spending tens of thousands of dollars.

I've tried so many of these additives over the years, with various results, and what I keep coming back to is simplicity. There's a few staples in the additive community that are worth considering. So although these additives aren't necessarily critical to maintaining longer engine life or efficiency, there are many cases where they really can make a difference.

Techron Complete Fuel System Cleaner

Starting with arguably the most trusted additive product on the market, Chevron's Techron: Complete Fuel System Cleaner. NOT to be confused with their "Injector Cleaner." The complete fuel system cleaner is one of the best there is. Simply pour it into the tank at the prescribed amount before filling, and it helps to clean everything in your fuel system, from fuel pumps and fuel level senders, to fuel pressure sensors and injectors. It even has the secondary benefit of being able to remove light carbon deposits from intake valves (on port injected engines) as well as from the tops of your pistons. Although there are much stronger solvents and cleaners you can pour in your gas tank, Chevron has done extensive "no harm" testing on it's fuel system cleaners, meaning it's essentially impossible to hurt anything while using their products, even when using it at much higher than prescribed ratios. I've personally used it at twice the prescribed amount when restoring a vehicle that's been sitting for many years, otherwise, simply using it at the regular dosage once a year or so can help keep varnish and deposits from forming in your fuel system, as well as keep your fuel injectors flowing freely. I've been told by a Chevron engineer not to use it before a road trip, or in such a case where you will drive through a tank of fuel quickly. He stated that the longer it's in the tank the better.

BMW Group Fuel System Cleaner Concentrate

This one is special. It's not a fuel system cleaner you're going to find at an auto parts store. This product was made by Chevron specifically for BMW, and it's only available at BMW dealerships. What makes this product truly amazing is the synergistic nature it has with the standard Techron Fuel System Cleaner mentioned above. This product is not to be added to your fuel tank! The way to use this product is by mixing it at about a 1:4 ratio with gasoline into a pressurized canister, and plumbing that canister directly into the fuel rail or high pressure pump of the vehicle. By first disabling the vehicle's electric in-tank fuel pump, and disconnecting the fuel line to the fuel rail or high pressure mechanical pump, the vehicle runs off this pressurized canister of Techron/fuel mixture until it runs out and the engine starts to sputter. The procedure is then repeated a second time with another full canister of the same mixture. After both canisters have been run through the engine, hook everything back up as it was, and add the black bottle of Techron (above) to the fuel tank at the prescribed ratio. The Techron in the fuel tank reacts with the residue leftover in injectors, intake valves (on port injected engines) and on the tops of pistons, and blows out more and more carbon deposits as you drive. Having performed this procedure on hundreds of BMW and Mini vehicles both port and direct-injected, I can tell you that the results are truly incredible. Smoother, quieter running engines, smoother idle, better acceleration, and more efficient. It's definitely a service worth performing every 50k miles or so.

Marvel Mystery Oil

Also known as "MMO" for short, Marvel Mystery Oil is somewhat of an anomaly of a product... hence the mystery I suppose. This product has been on the market since 1923 and is the only over the counter automotive product that is approved for aviation use. In my personal experience it's good at just about everything, but not truly great at any one thing. MMO can be added to fuel to aid in lubricity for fuel pumps, as well as acting as a fuel stabilizer. It's also a mild detergent, so it helps to keep things clean. MMO can also be added to your engine oil crankcase as a substitute for up to 25% of your engine oil. In the engine oil it claims to increase lubrication as well as act as a mild detergent and help to cut down on sludge and deposits. It's also known to help keep oil control rings on pistons clean, which may aid in keeping oil consumption in control. Another use case for this product is what's known as a "piston soak." Some engine designs are prone to clogging up the oil control rings on their pistons. MMO users have developed a procedure where, by removing spark plugs from the engine, you can add MMO directly to each cylinder using a small funnel. By letting the pistons "soak" for 24-48 hours, it is a means to clean up the oil control rings and drainback ports in the pistons. Although the smoke-show you get after starting your engine up after this procedure may terrify you, it does work. I've done it on my own vehicles that were drinking oil, as well as a few clients' cars. More often than not, there is a significant reduction in oil consumption.

SeaFoam

Another long-time favorite among the additive crowd is SeaFoam. Introduced in 1942, it's primary use case has been more of a cleaner than an additive, however since you can "add" it to both fuel and engine oil, we decided it deserved a spot in our lineup here. Most gearhead types think of SeaFoam as the cleaner you suck in to the intake system through a small vacuum line on the intake manifold in order to blast out all the carbon and deposits from intake valves and pistons. While still a viable option, they've now introduced a spray canister version of the original product, which is far more convenient, safer and easier to use for modern vehicles. Essentially the same thing in spray form, you spray the product in through a gap you create in the intake tube, where it attaches to the throttle body. Spraying the cleaner in this manner will clean up the throttle body, intake tract, intake valves, and pistons. This is a great maintenance service to do regularly on direct injected engines, where carbon buildup on the intake valves can become excessive. Another use case for SeaFoam is adding it to your fuel tank, where it can help clean up fuel system moisture and deposits, as well as intake valves (on port injected engines) and pistons. This product can also be added to engine oil as a type of engine flush. Not like MMO which is a lubricant and mild detergent, SeaFoam is a solvent, and is much stronger. Adding it to the engine oil at the prescribed amount, it states to only do so 100 to 300 miles before an oil change, since there is a risk of it freeing up a significant amount of deposits and sludge which can clog oil filters and supply orifices, as well as compromise the integrity of your engine oil. It's strong stuff, so be careful, but when used correctly it can make a noticeable difference in the way your engine runs.

Keep This in Mind About Additives

More is not usually better. There is no "mechanic in a can" or "magic bullet juice" you can pour into your fuel tank or engine oil that will fix a mechanical or electrical failure. Additives aren't meant to fix anything, although in some cases they can quiet your engine, increase fuel mileage, make them run a bit smoother, or reduce oil consumption.

Hopefully this article was of some value to you. This is by no means an exhaustive review of ALL the popular additives. I could see myself writing non-stop for months if I were to address each and every one.

I simply wanted to do an overview of the most popular and most widely used automotive additives. There's a reason why these ones are used every day by millions of DIYers and professionals alike.