If you have an older car that leaks or consumes oil, then you may be considering using an additive to help reduce or solve the problem. Many companies manufacture oil additives that claim some kind of benefit either by stopping a leak, providing better lubrication, or making the oil thicker to prevent oil burning or leaks. While many of the products won't harm your vehicle's engine, they also may not provide much benefit either. In some cases, you may make your problem worse.
Additives that "clean" the engine:
Many oil additives claim to clean sludge and other undesirable materials from the engine. There are also additives that prevent oil from foaming and decrease oxidation. These products promote a long life and higher performance due to the engine being cleaner and reducing friction from abrasives that are removed by the additives. The thing about these additives is that most quality motor oils already contain many of them. Adding more probably won't add any benefit, so it might better to save your money and pass on them.
Additives that "coat" the engine:
Some additives advertise that they coat and protect the engine in a way that even if the engine lost all its oil, it will still continue to run. The claim is that these additives cling to the parts and form a protective barrier. If you have a car that chronically loses a lot of oil, you may find this kind of product desirable because it could buy you some time if your car suddenly runs dry. There is no proof that these products protect the engine this way and running your car without oil can be disastrous. If you have a large oil leak, or any other condition where you are consuming a lot of oil, then have your engine repaired instead.
Additives that improve viscosity and stop leaks:
You may be considering an additive that thickens the oil or "restores" gaskets to solve your oil loss problems. The theory behind oil thickening additives is that thicker oils will have a harder time passing through worn rings into the combustion chamber or through leaky gaskets. Additives that are designed to stop leaks mostly do so by swelling the gaskets. In some cases, the additives may actually work, but only temporarily and could damage gaskets further. You will still need to have your engine or gaskets repaired eventually.
When it comes to oil additives, it may be better to save your money instead of buying them. All the additives you need should already be present in the oil you get during your oil change. Talk to your oil change technician to see what kinds of oils work best for your car. If you are having a problem where you car is losing or consuming large amounts of oil, have your car repaired instead.