20w-50 in 2010 Outback 2.5

I suspect the mechanic had a surplus of 20W50 to get rid of. My 2010 EJ25 runs great on 5W30. I could see moving up to a XW40, but the 20W50 is a bit extreme.
I'm not sure where exactly the OP is located, but they said about 7500' elevation. I checked the yearly average temps in Laramie just for grins. That's in southern Wyoming at 7165' elevation.

Average overnight lows are below 30F from mid-October to May 1. December, January, February and a few days on either side average below 22F; average daily highs are in the 30s for that same period.

5W-30 synthetic seems like an ideal oil for the car's location and duty cycle.
It will be fine to leave it in until late fall; just change it before colder winter weather sets in (late October/early November)