1. What kind of vehicle you have
2013 Subaru STI
2.5liter boxer 4cyl, IHI VF48 turbo at 17.5lbs peak boost
2. What your owner's manual says -- not just viscosity, but certifications (look for acronyms like API SM, ILSAC GF-4, etc.) and change intervals as well
5w30 synthetic, API SM or SN...