This topic has been on here several times, and on Edmunds as well. You will get answers going both ways...some say you will be eternally ****ed if you ever even think about it, some say as long as the same brand is used it is ok, some say ......
Personally I don't have a problem mixing oil wts, been doing it for years. As for what the final viscosity rating is you would have to do analysis to get the exact answer. I have been told by some of the oilco tech folks that it is not an exact mathematical weighted average but it is close enough. Keep in mind that even the same oil wt (eg 10w30,10w40,20w50) will exhibit a range of cold/hot actual viscosities between mfgrs at given reference temps. Pour/flash point/measured viscosity values can be different between mfgrs for the same marketed wt of oil in other words.
Some people are hung up on voiding warranties, other people are more concerned with taking vehicles through a longer lifetime,,,my vehicles haven't been in warranty for several years now, so I could care less about it, I care more about my vehicles "mature" years these days, under warranty I would just use what the mfgr says though. A new engine with under 36K miles is not the same as that same engine after 100-200k miles on it.
see ya
Rando