Galaxy clusters are important probes of structure growth and dark energy, whose abundance depends sensitively on cosmology. Well constrained cluster masses are required in order to reduce systematic uncertainties on cosmological parameters and allow cluster cosmology to be competitive. It is generally accepted that cluster masses determined assuming hydrostatic equilibrium are biased low by around 20 per cent. However, recent hydrodynamical simulations have shown that for the most massive clusters this bias increases to around 40 per cent. This increase can mainly be attributed to assuming a single temperature fit for the X-ray spectrum of the cluster, leading to a significant bias in the temperature profile. In traditional smoothed-particle hydrodynamics (SPH) there is no mixing so an equilibrium temperature is not reached between two gas layers which is not representative of a real cluster. In more modern SPH schemes, an artificial conductivity switch is included which allows for more realistic mixing but is numerically motivated. As such, we aim to investigate the effect of gas mixing on the X-ray mass bias by changing the SPH schemes in a representative sample of massive clusters.