A. The key factor in improving fuel economy is choosing the right viscosity grade. In comparison to an SAE 15W-40, SAE 10W-30, 5W-30 and 5W-40 oils now have a history of improving fuel economy as much as 4% in different types of operations. Most if not all 5W-40 or 5W-30 heavy-duty engine oils are marketed as full synthetic oils, whereas most 10W-30 oils are either conventional or synthetic blend oils. The key point is it is the lower viscosity, not the synthetic component that delivers the improvement in fuel efficiency.