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.