Depending on the age of the vehicle try to stick with
Subaru stuff.
If its a much older car, say 1990s model, early 2000,
you may use various brands or generic fluids that meet
specs, but more recent cars its better to use Subaru
branded stuff ...as newer cars seem more sensitive to
Subaru specific fluids.
The SUBARU ATF-HP for example is far superior to a
Valvoline ATF fluid specifying suitability for Subaru-HP,
in a my 2013 Forester XT. In my 2000 Impreza there is
no noticable difference in gear shifting smoothness
between the two, but in the Forester the difference
is big.
The Subaru oil filter is much preferable also.
Its tested for proper flow, and has correct bypass
valve setting (23 psi). If you look at some filters
that people use for their Subarus, the oil inlet
holes are smallish, example some Purolator oil
filters specified for Subaru. I am sure that given
certain conditions, say at startup or very high rpms
there has to be a difference in flow.
The Subaru green coolant for older cars is supposedly
also formulated to have more anti-corrossion properties
then regular green coolants. I don't use it, I use
regular green Preston, or Zerex, or Peak, but change
these every two years. The headgasket problems in boxer
engines are frequently blamed on coolant seeping under
gravity into vertically positioned head gaskets and
corroding them.
Subarus cars are a bit different tahn other cars and
its good to stick to Subaru branded stuff. Subaru fluids
are mostly made by Idemitsu so if you check carefully
you can buy this brand and it will probably be the same.
Beck Arnley brand also sells some fluids that are exactly
the same.
Basia