In this work, 32 different feedstocks formulated from four different stainless steel powders and two wax-based binder systems (paraffin wax and beeswax), for which the proportion of two different surfactant agents was varied from 0 to 5 vol.% (stearic acid and oleic acid), were obtained. The viscosity of the feedstocks according to the shear rate was measured using a rotational rheometer. The moldability of the mixtures was assessed using the injected length following real-scale injections. The results show that the influence of surfactant agents on viscosity depends on the main binder constituent used in the feedstock formulation. The moldability of paraffin wax–based feedstocks was significantly affected by the proportion of the surfactant agents, whereas that of beeswax-based feedstocks was not affected by the presence of stearic acid or oleic acid. It was confirmed that as little as 0.20 vol.% of stearic acid or oleic acid in paraffin wax–based feedstock is enough to produce the surfactant effect, leading to a significant increase in the moldability of feedstocks (e.g., fourfold longer injected length). This threshold concentration of surfactant agent in paraffin wax–based feedstocks was also established with different powder grades, shapes, and sizes. For feedstocks containing paraffin wax and surfactant agents, the presence of a discontinuity in the viscosity profiles was confirmed not to be an experimental artifact because the phenomenon was reproduced using three different measurement approaches.