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Should I Use Oil Additives?

By Katharine Swan
Updated May 23, 2024
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Engine oil additives are sometimes sarcastically referred to as “mechanic in a bottle.” This term refers to the marketing message, not to mention the belief shared by many people, that additives can improve gas mileage, increase horsepower, and fix other engine problems. In truth, they do offer limited benefits, but certainly no miracle cures.

For example, many companies make an oil additive that is supposed to stop leaks. This is an exaggeration of the truth. Engine seals often dry up, shrink, and crack with age, causing oil leaks to occur. Oil additives that are meant to stop these leaks are actually formulated to condition or re-moisturize these seals. The idea is that the conditioning will cause the seal to resume its original shape, therefore reversing the cause of the leak.

There are two problems with this theory. One, rubber seals that have already dried up and lost their shape will never be the same again, no matter how much reconditioning additives are used. Furthermore, no matter how well the seals are re-moisturized, cracks that have formed will always be there, necessitating replacement rather than a miracle cure.

The only way to cure leaks is via preventative maintenance. Oil additives that are used to prevent engine seals from drying out actually do work, because they keep the seals from cracking or losing their shape in the first place.

Another type of additive that is often found on the market is for cleaning sludge and buildup from the inside of an engine. These products often make a number of claims, for instance that they will increase the efficiency of the engine, boost gas mileage, and even increase horsepower. However, no matter how good these oil additives are, they cannot give the car horsepower or gas mileage that it did not have in the first place. Although these additives can in fact break down engine buildup, all this does is help to restore gas mileage and horsepower that has been lost as the engine has become impeded by sludge.

It is also important to note that although the detergents in these oil additives can break down the sludge on the inside of the engine, they cannot get it as clean as it was when it was new, or even as clean as a mechanic could get it by taking the engine apart and cleaning it. Because these oil additives have limited cleaning abilities, they are again best used regularly as preventative measures, to prevent vast amounts of buildup from occurring in the first place.

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Discussion Comments

By anon927458 — On Jan 24, 2014

Well, TOHO9000 did cause a difference in my car; it made the engine freeze up like winter in the Arctic. Damn, when will I ever learn not to take advice from the internet!

By Engineoil — On Oct 13, 2012

If you want to see the difference in your vehicle like car, truck or boat, then TOHO9000 is a High Performance Engine Oil Treatment.

By anon47503 — On Oct 05, 2009

The only oil additive I suggest/recommend is Seafoam, every 10,000 miles, after the second treatment. I've outlined my little process of "success" in the oil flush comments, but basically, you flush with seafoam for 20-30 minutes, change the lube, oil and filter (either yourself, or at a mechanic garage/shop). After the LOF (that's lube oil filter) change, pop the trunk in the parking lot, and add more seafoam, 1.5 fl. oz. per each quart of oil your crankcase holds (4.5 quarts of oil? Add anywhere from 6 to 6.75 fl oz. of seafoam for example - probably easier to measure a flat 6 fl. oz though). Drive the vehicle as normal, checking the oil every time you fill up for oil color discoloration. If the oil looks dirty, it needs to be changed. Don't drive any more than 3000 miles on this "seafoamed" oil.

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