Are they manual or automatics?
The manual shares fluid with the MT and front diff. Mobil 1 is not recommended for the MT, as the chemistry does not play nice with the metal used in the Subaru.
It is fine however with the front diff of the automatic, as they are separate units.
it is usually the friction modifiers that are wacky. you want FM in a diff.. and dont want any in a MT because they need friction for the shifting to be smooth etc.
the front diff on a MT needs several things which are in opposition.
GL-5 for hypoid gear protection (bad for MT)
FM for fuel economy (cant have with MT)
MT wants no FM's and GL-4 additive levels
so you have to live with GL-5 to protect the gears
and no FM so the MT shifts right.
Not sure who thought it was a good idea to have a combo unit but it think its not good engineering.
also if the front hypoid style diff gears go bad.. due to running gl-4 etc, you pretty much have to replace the whole transmission as just replacing the gears is REALLY expensive.
If you can find a gl-5 with the wet clutch jaso-mt? spec thats usually a step in the right direction for these as it means little or no FM's