As previously mentioned, the Subaru FB20 in the Crosstrek is primarily designed to run on synthetic engine oil in the SAE viscosity of 0W-20.
Any fully synthetic motor oil meeting the following specs should be used.
API (American Petroleum Institute) classification
SM with the words "ENGERGY CONSERVING" or SN with the words "RESOURCE CONSERVING"
ILSAC (International Lubricant Specification Advisory Committee)
GF-4 or GF-5, which can be identified with the ILSAC certification mark (Starburst mark)
meeting the SAE (Society of Automotive Engineers) engine oil viscosity rating of
0W-20 (if vehicle is used in North America of a North American country's specifications)
SAE viscosities 5W-30 and 5W-40 may also be used, meeting the above API or ILSAC specs, but Subaru recommends 0W-20.
The exact wording in my owner's manual is shown below for substitute engine oil.
If 0W-20 synthetic oil is not available, 5W-30 or 5W-40 conventional oil may be used if replenishment is needed but should be changed to 0W-20 synthetic oil at the next oil change.
Generally, any synthetic meeting the primary specs should do perfectly fine. I run the Idemitsu-manufactured Subaru Synthetic Motor Oil and my used engine oil analysis results have been fine so far with it.
You can also get a slightly-cheaper Idemitsu SAE 0W-20 viscosity motor oil without the Subaru branding.
But otherwise, I'm unaware of anything else specific to the topic of an optimal engine oil in the FB20.