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Eutectic Al-13%Si alloys are widely used in the automotive industry for manufacturing components, such as pistons and cylinder heads. To reduce greenhouse gas emissions and enhance the engine efficiency, their service temperature keeps increasing to 250-350 °C, leading to the deterioration of their mechanical properties and the creep resistance. In the present work, Mo was further added to Mn-containing Al-13%Si piston alloys aiming at improving the overall properties at elevated temperatures. Compared with the Mn-containing base alloy, Mo further enhanced the precipitation of dispersoids by expanding the dispersoid zone and restricting the dispersoid-free zone after the proper precipitation treatment (520 °C/12 h), resulting in a remarkable improvement in the yield strength at both room temperature and 300 °C, as well as the creep resistance at 300 °C. Furthermore, the beneficial effect of Mo addition on the improved yield strength and creep resistance was especially prominent during long-term thermal exposure (up to 1000 h at 300 °C) due to the synergistic effect of thermally stable dispersoids and the retardation of the gradual fragmentation and spheroidization of Si particles.