"Are fuel-saving oils a myth or reality? I just switched from a mineral 10W40 oil to a synthetic 5W30 oil, thinking that this would allow for a much longer drain interval. But what about the fuel-saving characteristics? The basic logic is that the lower the viscosity, the lesser the amount of friction between the metals. But do these fuel-saving oils really deliver what they promise? What is the percentage of fuel that can be saved by using these oils?"

Fuel-economy oils have become popular over the last several years, especially the 0W20 and 5W20 oils. Most of these oils will provide at least a 1- to 2-percent increase in fuel economy over using the next viscosity grade. You see a savings at the pump by switching from a 10W40 to a 5W30 because you are dropping a viscosity grade and will have less fluid friction. On the other hand, you have now lowered the viscosity and could potentially increase wear.

If you achieve a 1-percent savings with a vehicle that gets 40 miles per gallon (mpg), you have gained an extra 0.4 mpg. On a vehicle that has a 15-gallon tank, this equals an extra 6 miles per 15 gallons. If you figure that gallon of gas costs $2.50, the extra 6 miles converts to a savings of 45 cents. Based on an oil change interval of 10,000 miles, you have saved $6 dollars. Of course, these numbers will fluctuate based on fuel prices and the increase you actually see in using a lower viscosity oil.

Keep in mind that newer engines have tighter tolerances and better surface finishes. By using these lower viscosity oils with a cleaner engine design, you can optimize fuel economy. On older vehicles where the recommendation is a 5W30 or 10W30 oil, it would not be advised to switch to a lower viscosity oil. By decreasing the viscosity, you have the potential of added wear generation. With increased wear, you can expect engine life to decrease and performance to degrade. Once performance starts to fall off, your fuel mileage will likely decrease as well.

There are many variables to consider when switching to a fuel-economy oil, including how the vehicle will be used. If you have an older engine and your vehicle will be used in extreme temperature changes, for towing or for frequent heavy loads, then a 5W20 oil may not be the best choice. If you are looking to save money at the pump, try checking the tire pressure regularly and adjusting your driving habits.