Subaru allowed for the use of a 10W-40 in your temperature range so I'd just run it straight.
This would be Mobil 1 Turbo Diesel Truck, which is a dual rated oil.
Part of Subrau lore is that they need something thicker than a 5w30, even the NAs.
I'm not sure that this is true, but you'd have no problems running Mobil 1 Turbo Diesel Truck 5W-40 in Orlando.
If you want to mix it, go ahead, but I don't think you'd need to.
You might have some loss in fuel economy running this very thick oil (thicker than Rotella 5W-40) but given the laughable fuel economy these things have to begin with, I doubt that you'll notice.