This study investigates the influence of Fischer–Tropsch synthetic wax (FTW) on the compactability and rheological behaviour of conventional 50/70 and polymer-modified PMB 45/80–55 binders in warm mix asphalt (WMA) technology. The results demonstrate a clear binder-specific and concentration-dependent response rather than a linear improvement in workability. For the 50/70 binder, dynamic shear rheometer (DSR) analysis in conjunction with Black space diagrams enabled the identification of a critical FTW content of 4%, above which rheological instability and structural disturbances occur. The polymer-modified binder exhibited a synergistic interaction with FTW, whereby a low wax content (2%) partially mitigated the stiffness imparted by the polymer network, while a 3% addition enabled a statistically significant reduction in compaction temperature of approximately 20 °C without degradation of the SBS polymer structure. Low-temperature evaluation using the bending beam rheometer (BBR) revealed that, while FTW increases creep stiffness (S), it simultaneously reduces the stress relaxation capacity (m-value), particularly in the conventional binder matrix. In both binders, a pronounced rheological lock-up phenomenon was observed, characterized by a rapid increase in viscosity within the FTW melting range (90–115 °C), which defines a narrow technological window for effective compaction. Although FTW increased the high-temperature performance grade by approximately 20 °C, the BBR results indicate a trade-off between enhanced rutting resistance and increased susceptibility to low-temperature cracking.