Final answer:
Switching from 15W-40 to 10W-30 oil can improve fuel economy by reducing engine friction; however, the exact benefit varies by vehicle and condition. Ethanol blends like E85 can reduce MPG due to lower energy content.
Step-by-step explanation:
The question is concerning the fuel economy benefit of switching motor oil grades from 15W-40 to 10W-30. Motor oils are formulated with different viscosities to perform optimally in various engine conditions and temperatures, and their selection can have an impact on the vehicle's fuel efficiency. While the specifics of the fuel economy improvement will depend on the vehicle's engine type and driving conditions, in general, using a lower viscosity oil like 10W-30 can reduce engine friction compared to a higher viscosity oil like 15W-40, potentially improving fuel economy. This is because lighter oils have less resistance to flow and therefore the engine doesn't have to work as hard to circulate the oil.
It's important to note that the actual percentage of fuel economy benefit can vary and should be evaluated in the context of the vehicle manufacturer's recommendations and the ambient temperature conditions in which the vehicle operates. As a general reference, fuel economy improvements from using a lower viscosity oil are typically modest but can be notable over time, especially in terms of cumulative fuel savings.
When considering alternative fuels like ethanol blends, the energy density of the fuel is a crucial factor. For example, E85 (85% ethanol) has lower energy content compared to conventional gasoline, potentially reducing miles per gallon by up to 30%. This reflects the trade-off between using cleaner-burning fuels with lower carbon emissions and achieving optimal fuel economy.