Using 0W40 instead of 0W20 oil on the 2013 2.5 OB

To oversimplify, the first number is cold viscosity (start up or severe cold). The second number is hot viscosity (operating temp). If the recommendation is 0w20, then you can substitute anything with the last number of 20. You could also do a 30 if you felt like it. But a 40 is diesel and high temp territory. No need for that. It wouldn't hurt anything other than your mpg, but there's no reason to do it unless you were driving through the desert uphill.

Reasonable substitutes for 0w20 would be a 5w20, 0w30, or 5w30. Anything over that would be pointless. The only reason they recommend 0w20 over the previous 5w30 is efficiency. It is simply to help with gas mileage.

Ehh. No need to blindly follow. Its not mother subaru, its soa trying to meet MPG standards. But 0w40 is pointless. You could safely use a 5w30 with no issues.