A fuel dispenser includes first and second cylinders adapted to contain respective first and second fluids, one being fuel and the other being a fuel additive such as a lubricant. First and second pistons in the cylinders displace the fluids through outlets and into a mixing line. The dispenser typically allows the volume of fluids displaced from each cylinder to be accurately controlled and adjusted in order to change the ratios. This is achieved by an adjustable aperture adapted to be opened and closed and optionally varied in degree of opening, typically by rotation of at least one of the components, to release fluid contained in the cylinder in front of the piston without passing the fluid into a mixing line. The adjustable aperture includes an aperture in a piston head, allowing the fluid in front of the piston to pass into or through the piston.