I'd use Shell Rotella T Synthetic 5W-40 from Wal Wart at $12.88 per gallon.
And I'd change it twice a year, unless you start to get in more long highway trips a few times a month, then a 9 to 12 month interval would probably be OK.
Since this oil has a better base stock,and a more robust additive package it should be better for a car that rarely gets the sort of beneficial sustained 20 minutes of higher speed drivng.
The API SL rating is acceptable for the Subaru engine, and the absence of the Starburst or GF-3 rating should be far less important than the increased protection the extra additives and viscosity provide.
The main reason for not having the Starburst and GF-3 is probably because of the the extra additives and viscosity.
"ROTELLA T Synthetic SAE 5W-40 is made with Shell’s XHVI Synthetic base stocks, which are Group III."
"ROTELLA T Synthetic is a universal oil, meeting API Service Categories CH-4 for diesel engines, and SL for gasoline engines. Where the engine manufacturer recommends oil meeting either one (or both) of these Service Categories, or earlier categories, ROTELLA T can be a good choice.
However, ROTELLA T does not meet all the requirements of ILSAC and API Certification Mark (starburst symbol) standards, sometimes specified for gasoline engine oils (in addition to API SL).
These additional standards deal with fuel economy, and also limit phosphorus content (an element in all engine oils, but usually at a higher concentration in diesel oils). Phosphorus can affect catalyst activity in some exhaust emission control systems.
If your manual specifies only API SL (or earlier designation), then the catalyst is likely not sensitive to higher levels of phosphorus. If it also specifies ILSAC standards, using ROTELLA T may risk some catalyst activity loss."
If the Rotella T Synthetic 5W-40 doesn't appeal to you, then I'd use the Motorcraft 5W-30, Chevron Supreme 5W-30, Havoline 5W-30, Castrol GTX 5W-30, Pennzoil 5W-30, Mobil Drive Clean 5W-30 oil every 4 to 6 months.
I'd take the Motorcraft 5W-30 as my first choice.