A study was conducted to assess the effect(s) of high oil concentration on the pressure-independent 'limiting' permeate flux and the ability of a thin film model to adequately predict the Aux in the treatment of synthetic and waste metal working fluids using the high-shear rotary ultrafiltration (HSRUF) system. Two distinct limiting flux regions were ascertained, in contrast to a discrete flux decline with concentration predicted from the thin film model. Additionally, widely different gel layer oil concentrations were predicted based on data from each discrete region. Thus, it was not possible to predict the occurrence of the two-phase limiting flux or the closest packed arrangement of emulsified oil droplets using a thin film model. The net effect of the unique 'wagon wheel' turbulence promoters was to enhance hydraulic turbulence in the HSRUF system; thus, the transition from laminar to turbulent hydraulic flow occurred at lower Re, in the HSRUF system than reported previously for a solid plate turbulence promoter. Further, it was demonstrated that the occurrence of the two-phase limiting Aux resulted in operating conditions under which high feed oil concentrations could be attained while maintaining acceptable limiting flux.