A pump for conveying a highly viscous fluid includes a casing with at least a first inlet and an outlet for the fluid, and an impeller for conveying the fluid from the inlet to the outlet. The impeller is arranged on a rotatable shaft for rotation around an axial direction, and includes a front shroud facing the first inlet of the pump. The casing includes a stationary impeller opening for receiving the front shroud of the impeller and has a diameter. The front shroud and the stationary impeller opening form a gap having a width in a radial direction perpendicular to the axial direction, and the ratio of the width of the gap and the diameter of the impeller opening is at least 0.0045.