I would like to think engine designers consider that at some eventual point – oil gets dirty (and is there forever).<O</OI think there is more to it than just the pump having to work harder. It can only work so hard before the relief valve opens up. The oil galleries and bearing clearances may be smaller in this engine due to the lower viscosity oil. Some areas may not get sufficient quantities of oil. Why take a chance? T
Mobil 1 0W-40 Viscosity Index 185 / Mobil 1 0W-20 Viscosity Index 173.<O</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</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).