There should be no warranty problem with 15W50. The generic Impreza owners manual recommends 5W30, 10W30 for higher minimum temperatures. These are to display the Energy Saving logo. Severe duty (such a trailer towing) allows for up to 20W50. Mobil 1 15W50 fits within the viscosity/temperature ranges listed in the manual. It is, however, not an energy saving oil. (As if staying in boost is energy saving!!).
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