Subaru 2000 Legacy Owner's Manual page 351

In choosing an oil, you want the proper quality and viscosity, as well
as one that will add to fuel economy. The following table lists the rec-
ommended viscosities and applicable temperatures.
When adding oil, different brands may be used together as long as
they are the same API classification and SAE viscosity as those rec-
ommended by SUBARU.
Engine oil viscosity (thickness) affects fuel economy. Oils of lower vis-
cosity provide better fuel economy. However, in hot weather, oil of
higher viscosity is required to properly lubricate the engine.
SAE viscosity numbr and applicable temperature
B Recommended grade and viscosity under severe driving
conditions
If the vehicle is used in desert areas, in areas with very high tempera-
tures, or used for heavy-duty applications such as a towing a trailer,
use of oil with the following grade and viscosities is recommended.
API classification SJ or SH: SAE viscosity No.: 30, 40, 10W-50,
20W-40, 20W-50
11-12
OM-H2756