wow i was told by valvoline to never mix viscosities. a 10w30 and 10w40 will not act like a 10w35 but part of the
oil will act like a 10w30 and part like 10w40, he went on to say you are better off mixing brands rather than viscosity.
who knows? I do agree way way way too many use 20w50 because they think they have a "race" car. 10W30 is about the sweat spot or what Roush recommends in their $20k crate engines which comes with a two year warranty.