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°F is fine with oil, not °C though.
The dealers over here provides full synthetics only in 0W20 and 5W40 for current year Prius models, not Prime though. Heck, blend and mineral oils...
On freeway straight ahead driving with both hands safely and temporarily off the steering wheel for seconds, does the Prius: a ) travels straight...
As suggested above this is likely a wheel (mis)alignment issue. Does the vehicle drifts or veers to one side on straight ahead driving at speed...
a Gen 4 supposedly comes with a double wishbone Rear suspension system. I would be rather surprise if there isn't any rear tie-rod kind of...
