I would like to think engine designers consider that at some eventual point – oil gets dirty (and is there forever).<O
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Mobil 1 0W-40 Viscosity Index 185 / Mobil 1 0W-20 Viscosity Index 173.<O
Chemically, both oils exceed Very High VI (>110) classification. Mechanically, I doubt the 0-40 would fail lubricity performance, correlating the Forester-T runs Syn/5-30. Insert-Make-Here run 0-20 to hit better 40ºC/100ºC ASTM specs, CAFE as alluded to.<O
In (practical) application, it makes no (oil related probability) difference whether in–warranty/off-warranty, you either run spec/non-spec – get the 0-20 (especially if you DIY).