In most modern cars, there's usually no need for oil or fuel additives. Most of the gasoline and oil already has detergents, boosters, and other additives to increase the performance of your car. Fuel additives may help certain problems with newer cars, such as moisture in the fuel. A substance called 'dry gas', or methyl alcohol, added to your tank may help your car burn off the water. Always consult a mechanic or your auto dealer before using any additive in your new car. Some substances may void all or part of the warranty. Older cars may sometimes benefit from fuel or oil additives. Switching to a higher-octane gasoline or using an anti-knock fuel supplement can usually help prevent knocking on acceleration. Oil additives for older cars can sometimes help with smoking and loss of horsepower due to engine wear. Contact a qualified mechanic in your area for more information on oil and fuel additives.