A Toyota mechanic from a different dealer from the one where we got our '06 Scion told me that if he got a Yaris or Scion XA, he'd put the Toyota full-synthetic 0W20 oil in it. That's what we did. I'm pretty sure he told me that the motor was almost exactly the same as the gas motor in the Prius. We never had any sort of engine issues by going lower viscosity than the recommended 5W30.
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View my fuel log 2015 Mirage DE 1.2 automatic: 43.0 mpg (US) ... 18.3 km/L ... 5.5 L/100 km ... 51.7 mpg (Imp)