Because the Toyota engineers already specified the heavier weight oils as used in ROW specs, but were then asked to compromise to improve fuel economy. Management probably posed this question to them, "how thin can we really go and most probably not run into lubrication issues". This was then...
So your criteria for a good oil is one that is low in detergents such that it doesn't carry the burnt hydrocarbons (gasoline residues) back to the oil filter. Dark oil does not mean that the oil is burnt.
Looks like the mystery oil additive actually diluted the 'Good" high pressure oil additives of Moly, Ca, Magnesium, Phos ands Zinc and also a slight dilution of the base viscosity.
I high quality 0W-30 is just fine and beats the 5W-30 if driving in very cold temps. Only negative of a wider range is that more VI's are used and this tend to breakdown over time. However the highest quality Group IV and V true synthetics use more stable polymer chains to begin with in the...
You realize that it was Mobil that filed the law suit against Castrol for claiming that the Castrol Group III mineral oils were not a true synethic, that only Group IV and V PAO and Esters were true synethics. Yes, Mobil lost the law suit as being too righteous, as no oils are pure as they all...