We consider the adsorption at an oil-water interface of spherical particles of two types. The first type has a homogeneous surface of uniform wettability. The second type, so-called "Janus" particles, has two surface regions of differing wettabilities. Homogeneous particles are strongly surface active but are not amphiphilic, whereas Janus particles are both surface active and amphiphilic. We present calculations to show how the particle amphiphilicity, tuned by variation of either the relative surfaces areas or the different wettabilities of the two surface regions on the particles, influences the strength of particle adsorption. Increasing the amphiphilicity of the particles produces a maximum of a 3-fold increase in surface activity for average contact angles around 90 degrees. Unlike homogeneous surface particles, Janus particles remain strongly surface active for average contact angles approaching either 0 or 180 degrees.