http://techinfo.subaru.com/proxy/69530/p...314ASTIS_19.pdf
You should use synthetic engine oil that meets the same requirements given for conventional engine oil. When using synthetic oil, you must use oil of the same classification, viscosity and grade shown in this Owner’s Manual. Refer to “Engine oil” F12-5. Also, you must follow the oil and filter changing intervals shown in the Warranty and Maintenance booklet.
http://techinfo.subaru.com/proxy/69530/p...314ASTIS_20.pdf
5W-30 synthetic oil is the required oil for optimum engine performance and protection. Conventional oil may be used if synthetic oil is unavailable.
*: 10W-30, 10W-40, 5W-30 or 5W-40 conventional oil may be used if replenishment is needed but should be changed to 5W-30 synthetic oil at the next oil change.
Application, application, application.
For purposes of this discussion, let's refer to "synthetic" the way the general public and oil company marketing departments refer to synthetic.
IMO, it does depend on the application like the above posters inferred and the EVO does require synthetic and came with Mobil1 from the factory. Yes, a completely different animal than a standard Ralliart. And while some WRX owners have had good luck with conventional oils at short intervals of 3k miles or so, Subaru even recommends synthetic for the older turbocharged cars that don't "require" it.
It doesn't make sense to get a 265 - 305 hp turbocharged four cylinder Evo or Subaru and try to run conventional oil in it. Some of the other turbos mentioned are fine running conventional. And people shouldn't assume what works fine for one application is fine for all applications.
-Dennis