I've been using around 5oz. of 2/3rds Marvel Mystery Oil & 1/3rd Lucas Upper Cylinder Lubricant every other fill-up or so in the Zuki. The SX4 has a 11.9 gallon capacity fuel tank.
I don't feel comfortable claiming a MPG increase, because it's so subjective, but I have noticed a 1.4 MPG decrease when not using this mix.
I am thinking about trying this locally made fuel additive called
E3 Fuels, next.