I personally think the slant six is better for mpg than the 318. But you sacrifice the power in exchange for mileage. To get the best of both worlds I would turbocharge the slant. Slants are tough and realiable. The long stroke is good for gas mileage. The power produced is in the low end. Tune it to this and you will get excellent mileage. The turbo is there when you want oomphhh. People have told me 30 mpg is not unheard of in a a body with a slant. Thats stock! So if you start off with a winnner its only going to get better from there. I think the only way to get power and MPG is with a small engine tuned for MPG with a turbo. Two extra cylinders are just going to need more gas to keep it turning.