pulled from the Impreza (.pdf) manual...
(This information has probably been mentioned on the site 1 million times. Itallics are my edit. Because I live in the desert, I feel it can not be stressed enough)
The proper viscosity helps vehicle get good cold and hot starting by reducing viscous friction and thus increasing cranking speed.
• When replenishing oil, it does not matter if the oil to be added is a different brand from that in the engine; however, use oil having the API classification and SAE viscosity No. designated by SUBARU.
• If vehicle is used in desert areas with very high temperatures or for other heavy duty applications, the following viscosity oils may be used: API classification: SL or SJ. ILSAC classification: GF-3. SAE Viscosity No.: 30, 40, 10W-50, 20W-40, 20W- 50