Subaru came out with all kinds off silliness with the FB and running EXCLUSIVELY their Genuine Subaru Synthetic 0W20. Even talking nonsense about thick oil molecules being too large to circulate properly. I dont have a N.A. FB OM to read the CAFE oil grade "warnings". We tried a 5W30 in a 2017 Crosstrek to quiet that noisy beast, it ran like a dog. Even topping off the wife's new '23 Crosstrek D.I motor with 10W30 - even with just 3/4 a qt - that got a bit boggy with a stick and low low offroad gearing. So back to the 20 grades. It really prefers it's mama's milk - the Idemitsu Subaru 0w20 over QS or Valvoline or AFE. These have sensitive HLA, low tension rings and light slipper-skirt pistons. I but will say the AVCS seems less sensitive to grade than a Totota VVTi system that I am experienced with. Run the oil choice, drive it. you will see what she needs. 0w40 is nuts on a low power non-turbo eco car not being tracked - or towing real hard. M1 0w40 FS was a good oil though.
In the past 30 years we have had over eight Subaru from Justy to SVX - but mainly many Impreza and Foresters - most with sticks.IN the 1990's I used to run Rotella triple protect 10w-30 with 1 qt of Formula Shell 5W30 in the EJ engines since the 5W30 conventional then was too wimpy (low HTHS in situ) so I do know where the Subaru thickies were born.
One last note, Subaru may have high volume oil pumps, but on a N.A. P.I. engine I don't think that volume is going through the engine** - its likely being bypassed at the pump. It's still is a 4 cylinder with typical (read tight) bearing clearances unless subaru has some voluminous oiling squirters and piddlers for those long chains and then the piston undersides. Using an electrical circuit analogy, high impedance will result into low current flow. -Ken
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**after the galleries are pressurised