Mothballs work great in any engine- you can get up to 17 extra mpg.
More performance tips:
Replace brake fluid and power steering fluid with Amsoil 5w30.
Always run your car a quart low on oil, it gives it better combustion.
Replace antifreeze with a 50/50 mix of Crisco and vinegar.
Keep only 15 psi in your tires during the winter - another extra 5 mpg.
Use honeybee honey as a premium car wax (clearcoat safe).
These are all great tips I've picked up over the years on various auto websites. They're on the internet, they must work...