Pretty sure the wrong viscosity oil could have some effect, but surely not nearly that much. Is it much colder there now?
That's a huge hit... I'm thinking… Colder temperatures + wrong oil + oil overfill That would do it...
The mileage is completely relative. It's really a matter of how much better you're doing compared to ICE-only vehicles. Depending on the size of their engines, they struggle with the same road and weather conditions. Meaning that 62mpg trip would also yield better results for a normal car if the driver knows what he or she is doing. Granted, the Prius sees a better jump because you have more opportunities to completely disengage the engine. But you catch my drift. Don't get caught up in the numbers.
Speaking for the Gen II, I've heard stories of mileage dropping 50% or more if the tech fills way past the line. Check the oil levels, I'm willing to bet there's too much.
