Good grief! 15W50 Mobil 1 in a Prius! Is he going to cross Australia in the summertime there? At least it is synthetic. It is wintertime in the USA. Maybe 5W30 for the wintertime?
What is the required oil for your Prius, FoxS? Check the owner's manual. Maybe a 5W30 or 5W20?
And I was under the impression that Exxon/Mobil had stopped making 15W50 Mobil 1. I don't know that for a fact. If they did stop making it, how old is that oil?
A person needs to check their owner's manual for what viscosity of oil to use. My guess Mark it would be a 5W30 or 5W20.
There are not that many gasoline engine vehicles that require 15W50 motor oil today.